Progress with novel pharmacological strategies for gastro-oesophageal reflux disease

A novel role for the extracellular matrix glycoprotein-Tenascin-X in gastric function

KEY POINTS

Tenascin X (TNX) functions in the extracellular matrix of skin and joints where it maintains correct intercellular connections and tissue architecture TNX is associated exclusively with vagal-afferent endings and some myenteric neurones in mouse and human stomach, respectively. TNX-deficient mice have accelerated gastric emptying and hypersensitivity of gastric vagal mechanoreceptors that can be normalized by an inhibitor of vagal-afferent sensitivity. Cultured nodose ganglion neurones showed no changes in response to capsaicin, cholecystokinin and potassium chloride in TNX-deficient mice. TNX-deficient patients have upper gastric dysfunction consistent with those in a mouse model. Our translational studies suggest that abnormal gastric sensory function may explain the upper gut symptoms present in TNX deficient patients, thus making it important to study gastric physiology. TNX deficiency should be evaluated routinely in patients with connective tissue abnormalities, which will enable a better understanding of its role and allow targeted treatment. For example, inhibitors of vagal afferents-baclofen could be beneficial in patients. These hypotheses need confirmation via targeted clinical trials.

ABSTRACT

Tenascin-X (TNX) is a glycoprotein that regulates tissue structure via anti-adhesive interactions with collagen in the extracellular matrix. TNX deficiency causes a phenotype similar to hypermobility Ehlers-Danlos syndrome involving joint hypermobility, skin hyperelasticity, pain and gastrointestinal dysfunction. Previously, we have shown that TNX is required for neural control of the bowel by a specific subtype of mainly cholinergic enteric neurones and regulates sprouting and sensitivity of nociceptive sensory endings in mouse colon. These findings correlate with symptoms shown by TNX-deficient patients and mice. We aimed to identify whether TNX is similarly present in neural structures found in mouse and human gastric tissue. We then determined whether TNX has a functional role, specifically in gastric motor and sensory function and nodose ganglia neurones. We report that TNX was present in calretinin-immunoreactive extrinsic nerve endings in mouse and human stomach. TNX deficient mice had accelerated gastric emptying and markedly increased vagal afferent responses to gastric distension that could be rescued with GABAB receptor agonist. There were no changes in nodose ganglia excitability in TNX deficient mice, suggesting that vagal afferent responses are probably the result of altered peripheral mechanosensitivity. In TNXB-deficient patients, significantly greater symptoms of reflux, indigestion and abdominal pain were reported. In the present study, we report the first role for TNX in gastric function. Further studies are required in TNX deficient patients to determine whether symptoms can be relieved using GABAB agonists.

Digestive Endocrine Cell Numbers Contribute to Contraction/Relaxation of the Lower Esophageal Sphincter

It is often recommended that patients at risk for gastroesophageal reflux (GER) should remain in a seated or upright position for 30 to 60 min after eating. However, there is no scientific justification for this time frame. The lower esophageal sphincter (LES) is important for preventing GER, and some gastrointestinal hormones are known to regulate the LES. To obtain morpho-functional evidence for the time during which one should assume such a position, we used a rat model to focus on gastrin and cholecystokinin (CCK), the hormones that contract and relax the LES, respectively, and examined the changes in the number of cells immunoreactive to these hormones before and after eating. The present study is based on the deduction that vigorous secretion decreases the intracellular contents of hormones to a level lower than that detectable by immunohistochemistry. After 23 h of fasting (PC-0), rats were fed for 3 min and sacrificed 10 min (PC-10) and 60 min (PC-60) later. The number of gastrin cells was decreased in the PC-10 group; however, in the PC-60 group, the number had recovered to that of the PC-0 group. The number of CCK cells was unchanged in the PC-10 group but was decreased in the PC-60 group. Residual gastric contents were found in all groups, indicating that the LES is relaxed despite these contents and suggesting that there are some risks of gastroesophageal reflux even 60 min after eating.

The role of the 5-hydroxytryptamine pathway in reflux-induced esophageal mucosal injury in rats

BACKGROUND

Dysfunction of the 5-hydroxytryptamine (5-HT) signaling pathway can lead to gastrointestinal motility and secretion abnormalities and to visceral hypersensitivity. The aim of this study is to investigate the role of 5-HT in reflux-induced esophageal mucosal injury.

METHODS

Fifty 8-week-old male Sprague-Dawley (SD) rats were randomly divided into a gastroesophageal reflux (GER) model group (30 rats) and a sham surgery control group (20 rats). Four weeks after surgery, the esophageal mucosa was collected for histological evaluation, 5-HT concentrations, and 5-HT selective reuptake transporter (SERT) mRNA and 5-HT4 receptor (5-HT4R) protein expressions.

RESULTS

Twenty-seven rats in the GER model group survived, and three rats died. Histologically, in the GER model group, 20 rats had reflux esophagitis (RE group), and 7 rats had non-erosive reflux disease (NERD group). The 5-HT levels in the esophageal tissue from the RE group were significantly higher than those from the control and NERD groups. Both the RE and NERD groups showed significant increases in SERT mRNA expression of the esophageal mucosa than that of the controls, and the SERT mRNA level in the RE group was significantly higher than that in the NERD group. The 5-HT4R protein level of the esophageal mucosa in the RE group was significantly lower than that in the controls and the NERD group.

CONCLUSIONS

We conclude that a 5-HT signaling pathway disorder could be a major factor in the pathogenesis of GER and RE.

Treatment approaches of gastrointestinal dysfunction in Parkinson's disease, therapeutical options and future perspectives

Gastrointestinal (GI) dysfunction is a common but underestimated feature in Parkinson's disease (PD). Out of the multimodal spectrum of treatment options, there currently are only a few pharmacological treatments available to improve gastrointestinal motility and symptoms. Because enteric nervous function is mainly regulated by transmitters different from those involved in the brain, dopamine replacement is not a treatment option in PD patients. This article focuses on the known regulative mechanism of GI function and presents known and upcoming treatment options for GI dysfunction in PD.

Responses of human sling and clasp fibers to cholecystokinin (CCK) and gastrin through CCK receptors

BACKGROUND AND AIM

Cholecystokinin (CCK) and gastrin exert their influences via CCK receptors. This research was conducted to look at the responses of the sling and clasp fibers forming the human lower esophageal sphincter (LES) to CCK and gastrin, and the role of CCK receptors in the responses.

METHODS

Muscle strips of sling and clasp fibers from the LES were obtained from patients undergoing subtotal esophagectomy. Isometric tension responses of the strips to CCK-8 and gastrin-17 were studied, and the maximum effect (E(max)) for each agonist was derived. CCK-A receptor antagonist, CR1409 and CCK-B antagonist, CR2945 were applied to sling and clasp fibers and their pK(B) values were calculated.

RESULTS

Sling fibers produced significant contractions following exposure to CCK-8 and gastrin-17, while clasp fibers had less responses to the two agents. CR1409 and CR2945 inhibited responses of sling to CCK-8 in a concentration-dependent fashion. The inhibition effects of the two antagonists on clasp fibers were not measurable because there was a mild contraction of the fiber in response to CCK-8.

CONCLUSION

The contractions generated by sling fibers following exposure to CCK and gastrin are greater than that produced by clasp fibers. CCK-A receptors are more important for the generation of contractions by the sling fibers, whereas both CCK-A and CCK-B receptors are involved in the functional regulation of the clasp fibers. [Corrections added after online publication 28 April 2008: in the Background and Aims section of the preceding abstract, all instances of 'CKK' were corrected to 'CCK'. In the final sentence of the abstract 'CCKA'was corrected to 'CCK-A'. In the article title '(CKK)' was corrected to '(CCK)'.].

Role of cannabinoid receptors in the control of gastrointestinal motility and perception

The identification of endocannabinoids and cannabinoid CB1 receptors in key areas of the intestinal wall, such as cholinergic neurons, supports a role for cannabinoids in the control of gastrointestinal motility. Activation of CB1 receptors inhibits the peristaltic reflex and slows down gastrointestinal and colonic transit. Endocannabinoids play an important inhibitory role in the control of the occurrence of transient lower esophageal sphincter relaxations. Cannabinoid receptor agonists inhibit gastric emptying and intestinal motility in humans. There is strong anatomical support for a role of CB1 receptors in the control of gastrointestinal perception, since these receptors have been identified in key sites of the neuronal circuitry involved in the transmission of visceral pain. Experimental data indicate a visceral antinociceptive action of cannabinoid receptor agonists, which remains to be confirmed in humans.

Advances in roles of prokinetic agents in the treatment of gastroesophageal reflux disease

Gastroesophageal reflux disease (GERD) is characterized by the reflux of gastric content into esophagus with or without histological changes. Pathogenesis of GERD is lower esophageal sphincter (LES) dysfunction, abnormal clearing capacity of refluxed materials, delayed gastric emptying and abnormal resistance of esophageal mucosa to gastric acid, but the primary upper gastrointestinal motility dysfunction is regarded as the most important factor in general. Therefore, prokinetic agents which can restore gastric motility with increasing of LES and esophageal motility have been developed and used frequently in the treatment of GERD. There are several prokinetic agents such as metoclopramide, domperidone, cisapride and mosapride that facilitate acetylcholine release from the enteric cholinergic neurons through a selective 5-HT₄ receptor agonistic action. It is considered that prokinetic agents have the same effect as H2 blocker in the treatment of patients with mild GERD. Additionally, it was reported that a combination therapy with prokinetic agents and proton pump inhibitors (PPI) or H2 receptor antagonists (H2RAs) is more effective than monotherapy in patients with severe GERD. This article reviews the recent advances in the role of prokinetic agents in the treatment of gastroesophageal reflux disease.

Novel approaches to the pharmacological blockade of gastric acid secretion

Research into new methods of controlling acid secretion is driven by existing medical needs in gastro-oesophageal reflux disease treatment. Histamine receptor subtype 3 agonists offer one approach for acid inhibition but no agent is yet undergoing clinical testing. Other, as yet unrealized strategies include preventing the fusion of the tubulovesicular elements that contain H+/K+-ATPase with the parietal cell membrane, or blocking channels that recycle K+ in the parietal cell. Of more promise are gastrin (cholecystokinin) receptor antagonists and potassium-competitive acid blockers; examples of both are in clinical development. It is probable that gastrin receptor antagonists would be used adjunctively with proton pump inhibitors, possibly for meal-induced reflux. The potassium-competitive acid blockers have attributes that may facilitate use as monotherapy for the treatment of gastro-oesophageal reflux disease. The early promise of gastrin receptor antagonists and potassium-competitive acid blockers remains to be defined in large-scale trials.

Signals for nausea and emesis: Implications for models of upper gastrointestinal diseases

Nausea and vomiting are amongst the most common symptoms encountered in medicine as either symptoms of diseases or side effects of treatments. In a more biological setting they are also important components of an organism's defences against ingested toxins. Identification of treatments for nausea and vomiting and reduction of emetic liability of new therapies has largely relied on the use of animal models, and although such models have proven invaluable in identification of the anti-emetic effects of both 5-hydroxytryptamine(3) and neurokinin(1) receptor antagonists selection of appropriate models is still a matter of debate. The present paper focuses on a number of controversial issues and gaps in our knowledge in the study of the physiology of nausea and vomiting including: The choice of species for the study of emesis and the underlying behavioural (e.g. neophobia), anatomical (e.g. elongated, narrow abdominal oesophagus with reduced ability to shorten) and physiological (e.g. brainstem circuitry) mechanisms that explain the lack of a vomiting reflex in certain species (e.g. rats); The choice of response to measure (emesis[retching and vomiting], conditioned flavour avoidance or aversion, ingestion of clay[pica], plasma hormone levels[e.g. vasopressin], gastric dysrhythmias) and the relationship of these responses to those observed in humans and especially to the sensation of nausea; The stimulus coding of nausea and emesis by abdominal visceral afferents and especially the vagus-how do the afferents encode information for normal postprandial sensations, nausea and finally vomiting?; Understanding the central processing of signals for nausea and vomiting is particularly problematic in the light of observations that vomiting is more readily amenable to pharmacological treatment than is nausea, despite the assumption that nausea represents "low" intensity activation of pathways that can evoke vomiting when stimulated more intensely.

Systematic review: pathophysiology and management of gastrointestinal dysmotility in systemic sclerosis (scleroderma)

Gastrointestinal dysmotility in systemic sclerosis (scleroderma) is prevalent in 90% of patients, increasing morbidity and in some cases mortality. The resultant gastrointestinal complications are usually extensive, involving many regions of the gut from the oesophagus to the anus. Collagen replacement of vascular and enteric smooth muscle results in hypomotility, lumen dilatation, tensile rigidity and eventual loss of organ functions. The aim of this paper is to provide an overview of systemic sclerosis-related gastrointestinal dysmotility and available/potential therapeutic options. We evaluated published data on the pathophysiology and management of gastrointestinal dysmotility in systemic sclerosis patients using the MEDLINE database for English and non-English articles from 1966 to July 2005. Based on this systematic review, lifestyle and medical therapy approaches are preferred as they often improve and/or ameliorate symptoms. Surgery is only recommended with serious, rare complications such as bowel perforation or ischaemia. Alternative therapies such as acupuncture-based therapies are well tolerated, with clinical improvement and may be of potential therapeutic benefit for systemic sclerosis gastrointestinal dysmotility. Further elucidation of initiating and persistent mechanisms of systemic sclerosis-related gastrointestinal dysmotility will optimize the development of a multidisciplinary and more directed treatment regimen.

Clinical potential of GABAB receptor modulators

Metabotropic gamma-aminobutyric acid(B) (GABAB) receptors for the major inhibitory transmitter GABA, together with metabotropic glutamate (mGLuRs) receptors, the extracellular calcium-sensing receptors (CaSRs), some V2R pheromone receptors and T1R taste receptors, belong to the family of 3 G-protein-coupled receptors (GPCRs). GABAB receptors are known to control neuronal excitability and modulate synaptic neurotransmission, playing a very important role in many physiological activities. These receptors are widely expressed and distributed in the nervous system and have been implicated in a variety of neurodegenerative and pathophysiological disorders including epilepsy, spasticity, chronic pain, depression, schizophrenia and drug addiction. To form a functional receptor entity, GABAB receptors must exist as a heterodimer consisting of GABAB1 and GABAB2 receptor subtypes with two 7-transmembrane proteins, and these subunits arise from distinct genes. The GABAB1 subunit binds the endogenous ligand within its extracellular N-terminus, whilst the GABAB2 subunit is not only essential for the correct trafficking of the GABAB1 subunit to the cell surface, but is also responsible for the interaction of the receptor with its cognate G-protein. Allosteric modulation has recently been recognized as an alternative pharmacological approach to gain selectivity in drug action. It is now generally accepted that modulators acting at the allosteric sites provide a novel perspective for the development of subtype-selective agents acting at GPCRs. These agents interact with allosteric binding sites quite separate from the highly conserved agonist binding region. In this review, we present a new class of phenylalkylamines, based on the lead compound fendiline, that are potent positive potentiators of GABAB receptor-mediated function and discuss their putative clinical applications. It is proposed that these new modulators may have therapeutic value in GABAB receptor pharmacology and are capable of selectively modifying GABAB receptor function. The allosteric modulators are offering an attractive and novel means to identify new leads, that are devoid of side effects associated with GABAB receptor agonists, and may, therefore, represent a major advance in the drug discovery process.

Contemporary diagnostic and management techniques for extraesophageal reflux disease

PURPOSE OF REVIEW

To review recent advances in the diagnosis and treatment of extraesophageal reflux.

RECENT FINDINGS

For most patients, the diagnosis of extraesophageal reflux relies on history and laryngoscopic exam. The reliability and reproducibility of reporting these measures may be improved with validated symptom questionnaires and standardized scoring of physical exam findings. Though dual probe pH monitoring has been the gold standard for diagnosis, it does not measure non-acid reflux events. Intraluminal impedance monitoring has the capability of measuring all esophageal reflux events (liquid, solid, gas) and may be useful in the diagnosis of non-acid reflux. Proton pump inhibitors have replaced histamine receptor antagonists as the mainstay of treatment for extraesophageal reflux. Histamine receptor antagonists are used predominantly for nocturnal acid breakthrough, or step-down therapy. Promotility and cytoprotective agents are used less commonly. Baclofen is currently being evaluated for its ability to decrease the incidence of transient lower esophageal sphincter relaxations and reduce post-prandial acid and non-acid reflux events. For individuals refractory to medical therapy, laparoscopic fundoplication techniques have proven efficacy in relieving some symptoms; the long-term benefit is not yet known. There is yet no established data on the effects of endoluminal therapies on extraesophageal reflux symptoms.

SUMMARY

The diagnosis of extraesophageal reflux for most patients relies on history and laryngoscopic exam. The diagnosis can be further verified by dual probe pH and impedance monitoring. Proton pump inhibitors are the mainstay of treatment. Laparoscopic fundoplication is proven to relieve symptoms, but there is yet no data on the effects of endoluminal therapies on extraesophageal reflux symptoms.

Triptans and gastric accommodation: pharmacological and therapeutic aspects

In the past decade, several studies have reported a significant delay of gastric emptying induced by the anti-migraine agent sumatriptan (a 5-hydroxytryptamine (5-HT)1B/D receptor agonist) in healthy human beings. In patients with functional dyspepsia, sumatriptan improves gastric accommodation after food consumption and reduce perception of gastric distension, hence relieving epigastric symptoms. Recent studies have established that impaired accommodation after food consumption is a major patho-physiological mechanism in functional dyspepsia and restoration of accommodation is considered to be a potential therapeutic target. The precise site of action of sumatriptan in humans is at present unknown, although recent studies carried out using a canine model indicate that sumatriptan exerts its action on gastric accommodation through 5-HT1B receptors, since both GR127935 and SB216641 (respectively, non selective 5-HT1B/D and selective 5-HT1B receptor antagonists) fully antagonised the effects of sumatriptan. Gastric relaxation and enhanced accommodation to a distending stimulus seem to be a class effect of triptans, since it occurs not only with sumatriptan, but also with second-generation triptans (rizatriptan and naratriptan), at least in a canine model. In dyspeptic patients, administration of triptans would be able to restore gastric accommodation after a meal and to improve symptoms of early satiety, confirming the therapeutic potential of 5-HT1B/D receptor agonists in functional dyspepsia.