Endomorphin-1 and -2, endogenous ligands for the mu-opioid receptor, inhibit striated and smooth muscle contraction in the rat oesophagus

Purinergic inhibitory regulation of esophageal smooth muscle is mediated by P2Y receptors and ATP-dependent potassium channels in rats

Purines such as ATP are regulatory transmitters in motility of the gastrointestinal tract. The aims of this study were to propose functional roles of purinergic regulation of esophageal motility. An isolated segment of the rat esophagus was placed in an organ bath, and mechanical responses were recorded using a force transducer. Exogenous application of ATP (10-100 μM) evoked relaxation of the esophageal smooth muscle in a longitudinal direction under the condition of carbachol (1 μM) -induced precontraction. Pretreatment with a non-selective P2 receptor antagonist, suramin (500 μM), and a P2Y receptor antagonist, cibacron blue F3GA (200 μM), inhibited the ATP (100 μM) -induced relaxation, but a P2X receptor antagonist, pyridoxal phosphate-6-azophenyl-2,4-disulfonic acid (50 μM), did not affect it. A blocker of ATP-dependent potassium channels (KATP channels), glibenclamide (200 μM), inhibited the ATP-induced relaxation and application of an opener of KATP channels, nicorandil (50 μM), produced relaxation. The findings suggest that ATP is involved in inhibitory regulation of the longitudinal smooth muscle in the muscularis mucosae of the rat esophagus via activation of P2Y receptors and then opening of KATP channels.

ATP-Induced Contractile Response of Esophageal Smooth Muscle in Mice

The tunica muscularis of mammalian esophagi is composed of striated muscle and smooth muscle. Contraction of the esophageal striated muscle portion is mainly controlled by cholinergic neurons. On the other hand, smooth muscle contraction and relaxation are controlled not only by cholinergic components but also by non-cholinergic components in the esophagus. Adenosine triphosphate (ATP) is known to regulate smooth muscle contraction and relaxation in the gastrointestinal tract via purinergic receptors. However, the precise mechanism of purinergic regulation in the esophagus is still unclear. Therefore, the aim of the present study was to clarify the effects of ATP on the mechanical responses of the esophageal muscle in mice. An isolated segment of the mouse esophagus was placed in a Magnus's tube and longitudinal mechanical responses were recorded. Exogenous application of ATP induced contractile responses in the esophageal preparations. Tetrodotoxin, a blocker of voltage-dependent sodium channels in neurons and striated muscle, did not affect the ATP-induced contraction. The ATP-evoked contraction was blocked by pretreatment with suramin, a purinergic receptor antagonist. RT-PCR revealed the expression of mRNA of purinergic receptor genes in the mouse esophageal tissue. The findings suggest that purinergic signaling might regulate the motor activity of mouse esophageal smooth muscle.

Systematic Review With Meta-Analysis: Chronic Opioid Use Is Associated With Esophageal Dysmotility in Symptomatic Patients

INTRODUCTION

We aimed to conduct a systematic review and meta-analysis to assess the impact of chronic opioid exposure on esophageal motility in patients undergoing manometric evaluation.

METHODS

Multiple databases were searched through October 2022 for original studies comparing the manometric results of patients who have used chronic opioids (for >90 days) with those who do not. The primary outcomes were esophageal dysmotility disorders. Three high-resolution manometry parameters were conducted as secondary outcomes. A random-effects model was applied to calculate the odds ratio (OR) and means difference (MD) along with a 95% confidence interval (CI).

RESULTS

Nine studies were included in this meta-analysis. Opioid use was associated with higher esophageal dysmotility disorders, including distal esophageal spasm (pooled OR 4.84, 95% CI 1.60-14.63, P = 0.005, I 2 = 96%), esophagogastric junction outflow obstruction (pooled OR 5.13, 95% CI 2.11-12.43, P = 0.0003, I 2 = 93%), and type III achalasia (pooled OR 4.15, 95% CI 2.15-8.03, P < 0.0001, I 2 = 64%). No significant differences were observed for hypercontractile esophagus, type I achalasia, or type II achalasia. The basal lower esophageal sphincter pressure (MD 3.02, 95% CI 1.55-4.50, P < 0.0001, I 2 = 90%), integrated relaxation pressure (MD 2.51, 95% CI 1.56-3.46, P < 0.00001, I 2 = 99%), and distal contractile integral (MD 640.29, 95% CI 469.56-811.03, P < 0.00001, I 2 = 91%) significantly differed between the opioid use and nonopioid use group. However, opioid use was associated with a lower risk of ineffective esophageal motility (pooled OR 0.68, 95% CI 0.49-0.95, P = 0.02, I 2 = 53%).

DISCUSSION

Chronic opioid exposure is associated with an increased frequency esophageal dysmotility disorders. Our results revealed that opioid use is significantly associated with type III achalasia but not with type I and II achalasia. Therefore, opioid treatment should be taken into account as a potential underlying risk factor when diagnosing these major esophageal motor abnormalities.

Chronic opioid use is associated with obstructive and spastic disorders in the esophagus

BACKGROUND AND AIMS

Chronic opioid effects on the esophagus are poorly understood. We investigated whether opioids were associated with increased prevalence of esophageal motility disorders.

METHODS

A retrospective study of all patients undergoing high-resolution manometry (HREM) at the Yale Gastrointestinal Motility Lab between January 2014 and August 2019. Data were extracted from the electronic medical record after studies were reviewed by two motility specialists using the Chicago Classification v.3.0. We compared the manometric results of patients who use opioids to those who do not and adjusted for type and dose of opioids using a 24 h Morphine Milligram Equivalents (MME) scale to compare patients taking low or high amounts of opioids.

RESULTS

Four manometric abnormalities were significantly different between the opioid and non-opioid users. Achalasia type III, esophagogastric junction outflow obstruction (EGJOO), and distal esophageal spasm (DES) (p < 0.005, p < 0.01, and p < 0.005, respectively) were common among opioid users, whereas ineffective esophageal motility (IEM) was more common among non-opioid users (p < 0.01). The incidence of EGJOO was significantly higher in opioid users compared to non-opioid users (p < 0.001). Lastly, IRP, DCI, and distal latency were significantly different between the two groups. Patients in the high MME group had significantly greater IRP, DCI, and lower distal latency than non-opioids (p < 0.001). Also, achalasia type III and DES were more common in the high but not the low MME group.

CONCLUSIONS

Opioid use is associated with multiple abnormalities on esophageal motility and these effects may be dose-dependent.

Effects of codeine on esophageal peristalsis in humans using high resolution manometry

BACKGROUND AND AIM

Opioid receptors agonists have been demonstrated to impair lower esophageal sphincter (LES) relaxation and induce spastic esophageal dysmotility, but little was known for their impact on distension-induced secondary peristalsis. The aim of the study was to investigate the hypothesis whether acute administration of codeine can influence physiological characteristics of primary and secondary peristalsis in healthy adults.

METHODS

Eighteen healthy volunteers (13 men, mean age 27.5 years, aged 20-43 years) underwent high resolution manometry (HRM) with a catheter containing an injection port in mid-esophagus. Secondary peristalsis was performed with 10 and 20 mL rapid air injections. Two different sessions including acute administration of codeine (60 mg) or the placebo were randomly performed.

RESULTS

Codeine significantly increased 4-s integrated relaxation pressure (IRP-4s) (P = 0.003) and shortened distal latency (DL) (P = 0.003) of primary peristalsis. The IRP-4s of secondary peristalsis was also significantly higher after codeine than the placebo during air injections with 10 mL (P = 0.048) and 20 mL (P = 0.047). Codeine significantly increased the frequency of secondary peristalsis during air injections with 10 mL than the placebo (P = 0.007), but not for air injection with 20 mL (P = 0.305).

CONCLUSIONS

In addition to impair LES relaxation and reduce distal latency of primary peristalsis, codeine impairs LES relaxation of secondary peristalsis and increases secondary peristaltic frequency. Our study supports the notion in human esophagus that the impact of opioids on peristaltic physiology appears to be present in both primary and secondary peristalsis.

Codeine induces increased resistance at the esophagogastric junction but has no effect on motility and bolus flow in the pharynx and upper esophageal sphincter in healthy volunteers: A randomized, double-blind, placebo-controlled, cross-over trial

BACKGROUND

Chronic opioid use can induce esophageal dysfunction with symptoms resembling achalasia and a manometric pattern of esophagogastric junction-outflow obstruction (EGJ-OO). However, the effect of opioids in acute setting on pharyngeal function and esophageal body contractility has not been investigated.

METHODS

After positioning the high-resolution impedance manometry (HRiM) catheter, codeine (60 mg) or placebo (glucose syrup) was infused intragastrically. Forty-five minutes post-infusion, participants received liquid, semi-solid, and solid boluses to assess esophageal and pharyngeal function. HRiM analysis was performed adhering to the Chicago classification v3.0. (CC v3.0). Pressure flow analysis (PFA) for the esophageal body and the pharynx was performed using the SwallowGateway™ online platform.

KEY RESULTS

Nineteen healthy volunteers (HV) [5 male; age 38.3] were included. After codeine administration, higher integrated relaxation pressure 4 s values resulted in significantly reduced deglutitive EGJ relaxation and distal latency was significantly shorter. Distal contractility was similar in both conditions. Bolus flow resistance at the EGJ and distention pressures increased significantly after codeine infusion. Based on CC v3.0, acute infusion of codeine induced EGJ-OO in six HV (p = 0.0003 vs. placebo). Codeine administration induced no significant alterations in any of the pharyngeal PFA metrics.

CONCLUSIONS & INFERENCES

In HV, acute administration of codeine increased bolus resistance at the EGJ secondary to induced incomplete EGJ relaxation leading to major motility disorders in a subset of subjects including EGJ-OO. However, an acute single dose of codeine did not affect motility or bolus flow in pharynx and UES. ClinicalTrials.gov number, NCT03784105.

Opioids Interfere With Deglutitive Inhibition Assessed by Response to Multiple Rapid Swallows During High-Resolution Esophageal Manometry

INTRODUCTION

Normal response to multiple rapid swallows (MRS) during high-resolution esophageal manometry is deglutitive inhibition; opioids may interfere with this. The aim of this study was to evaluate the response to MRS in patients on opioids, not on opioids, and healthy controls.

METHODS

Response to MRS was evaluated for complete vs impaired inhibition in 72 chronic opioid users, 100 patients not on opioids, and 24 healthy controls.

RESULTS

Impaired deglutitive inhibition was significantly more frequent in chronic opioid users compared with patients not on opioids and healthy controls (54% vs 14% vs 0%; P < 0.0001).

DISCUSSION

Impaired deglutitive inhibition during MRS is frequent in opioid users, supporting that opioids interfere with esophageal inhibitory signals.

Opioid-Induced Esophageal Dysfunction: Differential Effects of Type and Dose

OBJECTIVE

Data regarding opioid effects on esophageal function are limited. We previously demonstrated an association between chronic opioid use and esophageal motor dysfunction characterized by esophagogastric junction outflow obstruction, distal esophageal spasm, achalasia type III, and possibly Jackhammer esophagus. Our aim was to characterize the influence of different opioids and doses on esophageal dysfunction.

METHODS

Retrospective review of 225 patients prescribed oxycodone, hydrocodone, or tramadol for >3 months, who completed high-resolution manometry from 2012 to 2017. Demographic and manometric data were extracted from a prospectively maintained motility database. Frequency of opioid-induced esophageal dysfunction (OIED, defined as distal esophageal spasm, esophagogastric junction outflow obstruction, achalasia type III, or Jackhammer esophagus on high-resolution manometry, was compared among different opioids. The total 24-hour opioid doses for oxycodone, hydrocodone, and tramadol were converted to a morphine equivalent for dose effect analysis.

RESULTS

OIED was present in 24% (55 of 225) of opioid users. OIED was significantly more prevalent with oxycodone or hydrocodone use compared with tramadol (31% vs 28% vs 12%, P = 0.0162), and for oxycodone alone vs oxycodone with acetaminophen (43% vs 21%, P = 0.0482). There was no difference in OIED for patients taking hydrocodone alone vs hydrocodone with acetaminophen. Patients with OIED were taking a higher median 24-hour opioid dose than those without OIED (45 vs 30 mg, P = 0.058).

DISCUSSION

OIED is more prevalent in patients taking oxycodone or hydrocodone compared with tramadol. There is greater likelihood of OIED developing with higher doses. Reducing the opioid dose or changing to tramadol may reduce OIED in opioid users.

Effects of remifentanil on esophageal and esophagogastric junction (EGJ) bolus transit in healthy volunteers using novel pressure-flow analysis

BACKGROUND

Remifentanil is associated with subjective dysphagia and an objective increase in aspiration risk. Studies of opioid effects have shown decreased lower esophageal sphincter relaxation. We assessed bolus transit through the esophagus and esophagogastric junction (EGJ) during remifentanil administration using objective pressure-flow analysis.

METHODS

Data from 11 healthy young participants (23±3 years, 7 M) were assessed for bolus flow through the esophagus and EGJ using high-resolution impedance manometry (Manoscan™, Sierra Scientific Instruments, Inc., LES Angeles, CA, USA) with 36 pressure and 18 impedance segments. Data were analyzed for esophageal pressure topography and pressure-flow analysis using custom Matlab analyses (Mathworks, Natick, USA). Paired t tests were performed with a P-value of < .05 regarded as significant.

KEY RESULTS

Duration of bolus flow through (remifentanil/R 3.0±0.3 vs baseline/B 5.0 ± 0.4 seconds; P < .001) and presence at the EGJ (R 5.1 ± 0.5 vs B 7.1 ± 0.5 seconds; P = .001) both decreased during remifentanil administration. Distal latency (R 5.2 ± 0.4 vs B 7.5 ± 0.2 seconds; P < .001) and distal esophageal distension-contraction latency (R 3.5 ± 0.1 vs B 4.7 ± 0.2 seconds; P < .001) were both reduced. Intrabolus pressures were increased in both the proximal (R 5.3 ± 0.9 vs B 2.6 ± 1.3 mm Hg; P = .01) and distal esophagus (R 8.6 ± 1.7 vs B 3.1 ± 0.8 mm Hg; P = .001). There was no evidence of increased esophageal bolus residue.

CONCLUSIONS AND INFERENCES

Remifentanil-induced effects were different for proximal and distal esophagus, with a reduced time for trans-sphincteric bolus flow at the EGJ, suggestive of central and peripheral μ-opioid agonism. There were no functional consequences in healthy subjects.

Serotonin-induced contractile responses of esophageal smooth muscle in the house musk shrew (Suncus murinus)

BACKGROUND

Serotonin (5-hydroxytryptamine, 5-HT) is a regulatory factor in motility of the gastrointestinal tract including the esophagus. Although we proposed that vagal cholinergic and mast cell-derived non-cholinergic components including serotonin coordinately shorten the esophagus, the precise mechanism of serotonin-induced contractions in the suncus esophagus is still unclear. Therefore, the aims of this study were to determine characteristics of contractile responses induced by serotonin and to identify 5-HT receptor subtypes responsible for regulating motility in the suncus esophagus.

METHODS

An isolated segment of the suncus esophagus was placed in an organ bath, and longitudinal or circular mechanical responses were recorded using a force transducer.

KEY RESULTS

Serotonin evoked contractile responses of the suncus esophagus in the longitudinal direction but not in the circular direction. Tetrodotoxin did not affect the serotonin-induced contractions. Pretreatment with a non-selective 5-HT receptor antagonist or double application of 5-HT₁ and 5-HT₂ receptor antagonists blocked the serotonin-induced contractions. 5-HT₁ and 5-HT₂ receptor agonists, but not a 5-HT₃ receptor agonist, evoked contractile responses in the suncus esophagus.

CONCLUSION & INFERENCES

The findings suggest that serotonin induces contractile responses of the longitudinal smooth muscle in the muscularis mucosae of the suncus esophagus that are mediated via 5-HT₁ and 5-HT₂ receptors on muscle cells. The serotonin-induced contractions might contribute to esophageal peristalsis and emetic response.

Opioid-Induced Esophageal Dysfunction (OIED) in Patients on Chronic Opioids

OBJECTIVES

Bowel dysfunction has been recognized as a predominant side effect of opioid use. Even though the effects of opioids on the stomach and small and large intestines have been well studied, there are limited data on opioid effects on esophageal function. The aim of this study was to compare esophageal pressure topography (EPT) of patients taking opioids at the time of the EPT (≤24 h) with chronic opioid users who were studied off opioid medications for at least 24 h using the Chicago classification v3.0.

METHODS

A retrospective review identified 121 chronic opioid users who completed EPT between March 2010 and August 2012. Demographic and manometric data were compared between the two groups using general linear models or χ(2).

RESULTS

Of the 121 chronic opioid users, 66 were studied on opioid medications (≤24 h) and 55 were studied off opioid medications for at least 24 h. Esophagogastric junction (EGJ) outflow obstruction was significantly more prevalent in patients using opioids within 24 h compared with those who did not (27% vs. 7%, P=0.004). Mean 4 s integrated relaxation pressure was also significantly higher in patients studied on opioids (10.71 vs. 6.6 mm Hg, P=0.025). Resting lower esophageal sphincter pressures tended to be higher on opioids (31.61 vs. 26.98 mm Hg, P=0.25). Distal latency was significantly lower in patients studied on opioids (6.15 vs. 6.74 s, P=0.044).

CONCLUSIONS

Opioid use within 24 h of EPT is associated with more frequent EGJ outflow obstruction and spastic peristalsis compared with when opioid use is stopped for at least 24 h before the study.

Physiology, signaling, and pharmacology of opioid receptors and their ligands in the gastrointestinal tract: current concepts and future perspectives

Opioid receptors are widely distributed in the human body and are crucially involved in numerous physiological processes. These include pain signaling in the central and the peripheral nervous system, reproduction, growth, respiration, and immunological response. Opioid receptors additionally play a major role in the gastrointestinal (GI) tract in physiological and pathophysiological conditions. This review discusses the physiology and pharmacology of the opioid system in the GI tract. We additionally focus on GI disorders and malfunctions, where pathophysiology involves the endogenous opioid system, such as opioid-induced bowel dysfunction, opioid-induced constipation or abdominal pain. Based on recent reports in the field of pharmacology and medicinal chemistry, we will also discuss the opportunities of targeting the opioid system, suggesting future treatment options for functional disorders and inflammatory states of the GI tract.

Engineering endomorphin drugs: state of the art

INTRODUCTION

Although endomorphins-1 (EM-1; H-Tyr-Pro-Phe-Trp-NH(2)) and -2 (EM-2; H-Tyr-Pro-Phe-Phe-NH(2)) are primarily considered agonists for the μ-opioid receptor (MOR), systematic alterations to specific residues provided antagonists and ligands with mixed μ/δ-opioid properties, suitable for application to health-related topics. While the application of endomorphins as antinociceptive agents and numerous biological endpoints were experimentally delineated in laboratory animals and in vitro, clinical use is currently absent. However, structural alterations provide enhanced stability; formation of MOR antagonists or mixed and dual μ/δ-acting ligands could find considerable therapeutic potential.

AREAS COVERED

This review attempts to succinctly provide insight on the development and bioactivity of endomorphin analogues during the past decade. Rational design approaches will focus on the engineering of endomorphin agonists, antagonists and mixed ligands for their application as a multi-target ligand.

EXPERT OPINION

Aside from alleviating pain, EM analogues open new horizons in the treatment of medical syndromes involving neural reward mechanisms and extraneural regulation effects on homeostasis. Highly selective MOR antagonists may be promising to reduce inflammation, attenuate addiction to drugs and excess consumption of high-caloric food, ameliorate alcoholism, affect the immune system and combat opioid bowel dysfunction.

Type 1 diabetes attenuates the modulatory effects of endomorphins on mouse colonic motility

Our previous studies have shown that endomorphins (EMs), endogenous ligands for mu-opioid receptor, display a significant potentiation effect on mouse colonic motility. In the present study, to assess whether diabetes alters these modulatory effects of EMs on colonic motility, we investigated the effects of EMs in type 1 diabetic mouse colon in vitro. At 4 weeks after the onset of diabetes, carbachol-induced contractions in the longitudinal muscle of distal colon were significantly reduced compared to those of non-diabetic mice. Furthermore, the contractile effects induced by EMs in the longitudinal muscle of distal colon and in the circular muscle of proximal colon were also significantly reduced by type 1 diabetes. It is noteworthy that EMs-induced longitudinal muscle contractions were not significantly affected by atropine, Nomega-nitro-l-arginine methylester (l-NAME), phentolamine, propranolol, hexamethonium, methysergide and naltrindole. On the other hand, tetrodotoxin, indomethacin, naloxone, beta-funaltrexamine, naloxonazine and nor-binaltorphimine completely abolished these effects. These mechanisms responsible for EMs-induced modulatory effects in type 1 diabetes were in good agreement with those of non-diabetes, indicating similar mechanisms in both diabetes and non-diabetes. At 8 weeks after the onset of diabetes, both carbachol- and EMs-induced longitudinal muscle contractions were similar to those of short-time (4 weeks) diabetic mice. In summary, all the results indicated that type 1 diabetes significantly attenuated the modulatory effects of EMs on the mouse colonic motility, but the mechanisms responsible for these effects were not significantly altered.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

In vitro characterization of the effects of endomorphin 1 and 2, endogenous ligands for μ-opioid receptors, on mouse colonic motility

The effects of endomorphin 1 (EM1) and 2 (EM2) in colonic motility remain unknown. We investigated the effects and mechanisms of these endomorphins (EMs) on the colonic motility in vitro by applying various neural blocking agents and various opioid receptor antagonists. EMs (10(-9) to 10(-6)M) displayed significant stimulatory effects on the basal tonus or spontaneous activity of mouse colon but not of stomach and small intestine. It is noteworthy that the contractile actions of EMs varied slightly among different regions of colonic longitudinal muscle layers, whereas the contractile responses induced by EMs were significantly different among different regions of circular muscle layers. EMs-induced longitudinal or circular muscle contractions were not significantly affected by atropine, N(G)-nitro-l-arginine methyl ester, phentolamine, propranolol and methysergide. Tetrodotoxin, indomethacin and naloxone completely abolished the EMs-induced colonic contractions. Surprisingly, EMs (10(-7)M)-induced longitudinal muscle contractions were significantly attenuated by nor-binaltorphimine (3x10(-6)M). By contrast, pretreatment with naltrindole (10(-6)M) did not significantly affect EMs-induced longitudinal or circular muscle contractions. Interestingly, the circular muscle contractions in response to EM2 (10(-7)M) were not fully blocked by beta-funaltrexamine (6x10(-6)M). Naloxonazine (10(-6)M) almost fully antagonized the EMs-induced longitudinal or circular muscle contractions, and these effects could be only partially reversed by extensive washing. All the results indicated that the mechanisms and sites of actions of EMs were region-specific. Furthermore, these findings showed that the activation of multiple subtypes of opioid receptors, possibly including mu(1) (naloxonazine-sensitive), mu(2) and even other forms of muORs (beta-FNA-insensitive), was required for EMs-induced mouse colonic motility.

In vitro Characterization of Novel Peptide Inhibitors of Endomorphin-degrading Enzymes in the Rat Brain

Endomorphins, endogenous mu-opioid receptor ligands, have been shown to exert antinociceptive, antidepressant, anxiolytic, and neuromodulatory effects, as well as to influence cardiovascular, respiratory, and gastrointestinal systems. In the present study, we designed and synthesized a series of tetrapeptides and tripeptides (amides and peptide acids) of similar to endomorphins structure, but with low mu-opioid receptor affinity, and tested them as possible inhibitors of endomorphin-degrading enzymes. The obtained results indicate that the tripeptides Tyr-Pro-Ala-NH2 and Tyr-Pro-Ala-OH, which do not bind to the mu-opioid receptors, are potent inhibitors of endomorphin-degrading enzymes in the rat brain. We suggest that the in vivo administration of these novel analogs may enhance physiological effects of endogenous endomorphins by decreasing the rate of their enzymatic cleavage.

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.

Tachykinins are involved in local reflex modulation of vagally mediated striated muscle contractions in the rat esophagus via tachykinin NK1 receptors

The objective of the present study was to investigate the hypothesis of the presence of a local neural reflex modulating the vagally mediated contractions of striated muscle in the rat esophagus and to determine the possible involvement of tachykinins in such a local neural reflex. Electrical stimulation of the vagus nerve evoked twitch contractile responses that were abolished by d-tubocurarine (5 microM). Capsaicin (1-100 microM) inhibited the vagally mediated twitch contractions o f the normal rat esophageal preparations concentration-dependently but not those of the neonatally capsaicin-treated ones. NG-nitro-L-arginine methyl ester (100 microM), a nitric oxide synthase inhibitor, blocked the inhibitory effect of capsaicin and exogenous application of a nitric oxide donor (1 mM) inhibited the vagally mediated twitch contractions. Capsaicin suppressed acetylcholine release from the normal rat esophageal segments evoked by vagus nerve stimulation but not that from the neonatally capsaicin-treated ones. A selective tachykinin NK1 receptor antagonist (0.1 or 1 microM) attenuated the inhibitory effect of capsaicin. However, antagonists of tachykinin NK2, tachykinin NK3 and calcitonin gene-related peptide receptors (1 microM) did not have any effect. A tachykinin NK1 receptor agonist (1 or 5 microM) inhibited the vagally mediated twitch contractions, which was prevented by NG-nitro-L-arginine methyl ester (100 microM). These data suggest that the rat esophagus might have a local neural reflex inhibiting the vagally mediated striated muscle motility, which consists of capsaicin-sensitive sensory neurons and myenteric nitrergic neurons, and that tachykinins might be involved in the neural reflex through tachykinin NK1 receptors.

Enteric co-innervation of motor endplates in the esophagus: state of the art ten years after

The existence of a distinct ganglionated myenteric plexus between the two layers of the striated tunica muscularis of the mammalian esophagus represented an enigma for quite a while. Although an enteric co-innervation of vagally innervated motor endplates in the esophagus has been repeatedly suggested, it was not possible until recently to demonstrate this dual innervation. Ten years ago, we were able to demonstrate that motor endplates in the rat esophagus receive a dual innervation from both vagal nerve fibers originating in the brain stem and from varicose enteric nerve fibers originating in the myenteric plexus. Since then, a considerable amount of data could be raised on enteric co-innervation and its occurrence in a variety of species, including humans, its neurochemistry, spatial relationships on motor endplates, ontogeny, and possible roles during esophageal peristalsis. These data underline the significance of this newly discovered innervation component, although its function is still largely unknown. The aim of this review is to summarize current knowledge about enteric co-innervation of esophageal striated muscle and to provide some hints as to its functional significance.

The role of endogenous opioids in the control of gastrointestinal motility: predictions from in vitro modelling

Gastrointestinal motility can be assessed in vitro by investigating the effects of drugs or gene knockouts on intestinal propulsion, and on neurone-mediated responses evoked by electrical field stimulation (EFS). The latter predominantly measure enteric motor activity and can detect prokinetic activity of exogenous agents. Some evidence suggests that naloxone has prokinetic activity when evaluated for an ability to modulate responses to EFS, but the effects are inconsistent across different species or intestinal regions. Models of intestinal peristalsis measure an integrated sensory-motor nerve function and possess more intact neuro-neuronal connections. In such preparations, the effects of naloxone also suggest a prokinetic property but again, this is inconsistent. By contrast, consistent prokinetic activity of naloxone is apparent in models where peristalsis is compromised by drug-induced suppression of motor nerve activity or by modulation of endogenous processes using receptor antagonists or inappropriate intraluminal distension. These data suggest that endogenous opioids play little or no role in normal intestinal physiology, but suppress intestinal motility when motor function is compromised. Consequently, drugs that antagonize opioid receptors may exert prokinetic activity in conditions where intestinal motility is reduced, such as constipation. Further work is required to elucidate the opiate receptor(s) involved.

Development of neuromuscular junctions in the mouse esophagus: Morphology suggests a role for enteric coinnervation during maturation of vagal myoneural contacts

The time course of establishment of motor endplates and the subsequent developmental changes in their enteric and vagal innervation were examined in esophageal striated muscle of perinatal and adult C57/Bl6 mice by using immunocytochemistry and confocal laser scanning microscopy. Nicotinic acetylcholine receptors were visualized with alpha-bungarotoxin; vagal motor nerve terminals with antisera against vesicular acetylcholine transporter; and enteric nerve fibers with antisera against neuronal nitric oxide synthase, vasoactive intestinal peptide, and galanin. Because the various stages of esophageal striated myogenesis advance caudocranially, i.e., more mature stages are found cranial to immature stages, longitudinal cryosections through the esophagus were investigated. Synaptogenesis was divided into several distinct stages. 1) Mononucleated cells express acetylcholine receptors over their entire surface. 2) They start to cluster receptors without nerve fiber contacts. 3) The first nerve contact on a growing receptor cluster is made by a vagal nerve terminal, followed by an enteric terminal. 4) Vagal terminals grow until they match the size of endplate areas, and one to three enteric terminals intertwine with them on every receptor cluster. 5) After vagal terminals have covered the whole endplate area, enteric terminals are withdrawn from the majority of motor endplates. In a minority of endplates, enteric coinnervation persists through adulthood. The enteric innervation of all developing motor endplates, shortly after vagal terminals have contacted them, and the removal of enteric nerve fibers from the majority of mature motor endplates suggest a major role of enteric nerve fibers during maturation of esophageal neuromuscular junctions.

Anatomical and neurochemical features of the extrinsic and intrinsic innervation of the striated muscle in the porcine esophagus: evidence for regional and species differences

Studies of the intrinsic and extrinsic innervation patterns of esophageal motor endplates (MEPs) are mainly confined to small rodents. Therefore, an immunocytochemical, denervation and tracing study was conducted on the pig, an experimental model in which the distribution of the striated esophageal muscle portion more closely resembles the human situation. The purpose of this study was to analyze the origin and neurochemical content of the nerve fibers participating in the myoneural synapse. Fifteen 6-week-old domestic pigs were studied by immunohistochemistry combined with alpha-bungarotoxin labeling to define the co-innervation patterns of nitrergic and peptidergic nerve terminals in MEPs. Some animals were subjected to unilateral infra- or supranodose vagotomy to determine the origin of the nerve terminals in MEPs. Special attention was paid to the interregional differences in terms of co-innervation rates, and these findings were compared with literature data on small mammals. Double stainings revealed that most of the nNOS-immunoreactive (ir) terminals in MEPs co-stained for VIP, GAL and NPY, but not for PACAP and L-ENK. PACAP- and L-ENK-ir terminals were coarser than nNOS-ir terminals, and largely co-localized VAChT. High percentages of MEPs at the cervical level were contacted by PACAP- (approximately 94%) and L-ENK-ir (approximately 78%) terminals, but the proportion of both decreased in the rostrocaudal direction. Vagotomy significantly reduced their presence in MEPs at the thoracic and abdominal levels, while nNOS-ir terminals observed in approximately 30% of the MEPs were unaffected by vagotomy. Immunostainings on brainstem cryosections after retrograde tracing from the cervical esophagus showed that a large number of FB-positive cells in the nucleus ambiguus were PACAP-ir (approximately 72%). C-kit-positive interstitial cells of Cajal were seen adjacent to the striated muscle fibers, apparently without direct relationship to MEPs. Similar to mouse esophagus, intrinsic nitrergic fibers were found to run close to, or even spiral around, these interstitial cells, an association that might point to a role as specialized spindle proprioceptors. In conclusion, the cholinergic terminals-part of which coexpress PACAP and/or L-ENK-that innervate MEPs in the porcine esophagus have a vagal origin, whereas the nNOS/VIP/GAL/NPY-ir fibers co-innervating these MEPs are intrinsic in nature. The regional differences observed along the esophageal length pertain to the neurochemical content of the vagal motor innervation of the MEPs.

Quantitative detection of atropine-delayed gastric emptying in the horse by the 13C-octanoic acid breath test

The 13C-octanoic acid breath test has been correlated significantly to radioscintigraphy for measurement of gastric emptying indices in healthy horses. The objective of this study was to investigate the validity of the test for measurement of equine delayed gastric emptying, prior to its potential clinical application for this purpose. A model of atropine-induced gastroparesis was used. Gastric emptying rate was measured twice in 8 horses using concurrent radioscintigraphy and/or breath test after treatment i.v. with either atropine (0.035 mg/kg bwt) or saline in randomised order. Analysis of both data sets demonstrated that the atropine treatment had caused a significant delay in gastric emptying rate. Paired breath test data showed an atropine-induced delay in gastric half-emptying time (t 1/2), with no overlap in the 99% CI range (P < 0.001). Significant correlations were found between scintigraphy and 13C-octanoic acid breath test for calculation of both t 1/2 (P < 0.01) and lag phase duration (P < 0.05) in the atropine-delayed emptying results. The mean (s.d.) bias in breath test t 1/2 when compared with scintigraphy was 1.78 (0.58) h. The results demonstrated that the 13C-octanoic acid breath test was an effective diagnostic modality for the measurement of equine delayed gastric emptying. The technique offers advantages to existing methods for clinical investigation, as it is noninvasive, not radioactive, quantitative and requires minimal equipment or training to perform.

Endomorphins 1 and 2 reduce relaxant non-adrenergic, non-cholinergic neurotransmission in rat gastric fundus

It is now well established that opioids modulate cholinergic excitatory neurotransmission in the gastrointestinal tract. The aim of the present study was to characterize a possible effect of endomorphins on nonadrenergic, noncholinergic (NANC) relaxant neurotransmission in the rat gastric fundus in vitro. The drugs used in the experiments were the endogenous mu-opioid receptors (MORs) endomorphin 1 and 2 and the mu-opioid receptor antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2). CTAP left the basal tonus and the spontaneous activity of the preparation unchanged. Electrical field stimulation (EFS) under NANC conditions at frequencies ranging from 0.5 to 16 Hz caused a frequency-dependent relaxant response on the 5-hydoxytryptamine (5-HT) (10(-7) M) precontracted smooth-muscle strip. Both endomorphin 1 and endomorphin 2 significantly reduced this relaxation in a concentration-dependent manner. Endomorphin 1 proved to be more potent in reducing the relaxant responses. The endomorphin effects were significantly reversed by the MOR antagonist CTAP. CTAP itself did not influence the EFS-induced relaxation. In summary, these data provide evidence that the endogenous MOR agonists endomorphin 1 and 2 can reduce nonadrenergic, noncholinergic neurotransmission in the rat gastric fundus smooth muscle via a pathway involving MORs. The physiological relevance of these findings remains to be established, since the data presented suggest that the endomorphins act as neuromodulators within NANC relaxant neurotransmission.

Effects of endomorphin-1 and -2 on mu-opioid receptors in myenteric neurons and in the peristaltic reflex in rat small intestine

1. The aim of the present investigations was to characterize the effect of endomorphins on the function of rat small intestine smooth muscle and on the electrically induced ascending and descending reflex pathway of rat small intestine in vitro. 2. Endomorphin-1 and -2 left the basal tonus and the pharmacologically stimulated smooth muscle unchanged. In contrast, electrically induced twitch contractions were significantly reduced by endomorphin-1 and -2 and this reduction was reversed by the mu-opioid receptor antagonist Cys-Tyr-Orn-Pen-amide (CTOP), suggesting a specific mu-opioid receptor-mediated effect on neural tissue. 3. In the reflex model, endomorphin-1 caused a significant inhibition (IC50 2.3 x 10(-8) mol/L) of the ascending contraction (10(-8) mol/L: -28.3 +/- 5.8%; 10(-7) mol/L: -94.7 +/- 0.2.8%; both P < 0.05; n = 7). Descending relaxation increased at a concentration of 10(-8) mol/L endomorphin-1 (+61.6 +/- 24.5%; 10(-7) mol/L: +237.0 +/- 65.4%; both P < 0.05; n = 6). 4. Endomorphin-1 caused a further significant increase in the latency of the ascending contraction (10(-8) mol/L: +44.7 +/- 20.5%; 10(-7) mol/L: +93.5 +/- 16.1%; both P < 0.05; n = 7), whereas the latency of the descending relaxation was unaltered (n = 7). Similar results were observed for endomorphin-2. 5. All effects could be reversed by a wash-out afterwards and were blocked by pre-incubation with CTOP (10-6 mol/L). 6. Reverse transcription-polymerase chain reaction demonstrated mRNA expression of mu-opioid receptors in the rat ileum longitudinal muscle/myenteric plexus preparation, as well as in the oesophagus and stomach. 7. Endomorphin-1 and -2 reduce the cholinergic-induced contractile response of the rat ileal smooth muscle preparation via a presynaptic mechanism. 8. By a specific and reversible interaction with mu-opioid receptors, the ascending excitatory and descending inhibitory reflex responses were attenuated or facilitated, respectively. 9. In conclusion, the endomorphins may be the physiological endogenous mu-opioid receptor agonists in the rat small intestine.

Characterization of vagal input to the rat esophageal muscle

There is recent morphological evidence for an interaction of autonomic nerve fibers and extrinsic motor nerves of the rat esophagus. The aim of the present study was to investigate a possible functional role of this autonomic innervation of vagal motor fibers on rat esophageal smooth and striated muscle function in vitro. The entire esophagus with both Nn vagi, including the Nn recurrentes, was dissected and placed in an organ bath with oxygenated Krebs-Ringer buffer. Contractile activity was measured in longitudinal direction with a force transducer. Both Nn vagi were placed on a bipolar platinum electrode 2 cm apart from the esophagus. Vagal stimulation, applied for 1 s (40 V, 0.5 ms, 20 Hz) resulted in a biphasic contractile response, which was completely blocked by tetrodotoxin (10(-6) M). The first part consisted of a tetanic striated muscle contraction, which was abolished by tubocurarin (10(-5) M) but unaffected by atropine (10(-6) M) or hexamethonium (10(-4) M). In contrast, the second part was completely abolished by hexamethonium (10(-4) M) and atropine (10(-6) M), whereas tubocurarine (10(-5) M) showed no influence, suggesting a stimulation of preganglionic nerve fibers supplying esophageal smooth muscle (muscularis mucosae). In order to characterize possible autonomic transmitters of the ENS of the esophagus, the following experiments were carried out. The magnitude of the striated muscle response was unaffected by VIP (10(-7) M), 5-HT (10(-6) M) and galanin (10(-8) - 10(-7) M), whereas they caused an inhibition of the smooth muscle response (VIP: -53.8 +/- 4.2%; galanin 10(-8) M: - 18.5 +/- 2.2%; 10(-7) M: -40.4 +/- 2.9%; 5-HT: -78.2 +/- 2.1%). The inhibitory effects of VIP and galanin on smooth muscle were reversible by the antagonists VIP 10-28 and galanin 1-15. In the presence of the nitric oxide synthase (NOS) inhibitor L-NNA (10(-4) M), the smooth and striated muscle contraction were not significantly influenced. Exogenous application of the NO-donor DEA-NO (10(-4) M) reduced the smooth muscle contraction by -81.6 +/- 7.4%, but had no significant effect on the striated muscle contraction. Though immunohistochemical findings are highly suggestive of an nitrergic autonomic modulation of striated muscle contraction by enteric neurons, we could not demonstrate a NO-mediated action on striated muscle activity. Therefore, the physiological relevance of the immunohistochemical findings remain unclear.