5-HT(4) receptors on cholinergic nerves involved in contractility of canine and human large intestine longitudinal muscle

The impact of long-course chemoradiotherapy on the myenteric plexus, neuromuscular functions and responses to prokinetic drugs in the human rectum

BACKGROUND & AIMS

The long-term effects of chemoradiotherapy on human rectum are poorly understood. The aims were to investigate changes in inflammatory status, myenteric neuron numbers/phenotype, neuromuscular functions and prokinetic drug efficacy.

METHODS

Macroscopically normal proximal-to-mid rectum was obtained from 21 patients undergoing surgery for bowel cancer, 98 days (range: 63-350) after concurrent capecitabine and pelvic radiotherapy, and 19 patients without chemoradiotherapy. Inflammatory status was measured by H&E, CD45 staining and qPCR. Myenteric neurons were examined by immunohistochemistry. Neuromuscular functions and drug efficacy were studied using exogenous agents and electrical field stimulation (EFS) to activate intrinsic nerves.

RESULTS

Inflammation was not detected. Numbers of myenteric ganglia/neurons were unchanged (11.7 ± 2.4 vs. 10.3 ± 2.2 neurons/mm myenteric plexus with/without chemoradiotherapy) as were the numbers of cholinergic/nitrergic neurons. EFS stimulated cholinergic and nitrergic neurons so the contractile response of the muscle was the sum of both but dominated by cholinergic (causing contraction) or less often, nitrergic activity (relaxation), followed, after termination of EFS, by neuronally mediated contraction. Inhibition of nitric oxide synthase (by L-NAME 300 μM) more clearly defined EFS-evoked contractions. The 5-HT4 agonist prucalopride 10 μM and the cholinesterase inhibitor donepezil 1 µM, respectively increased and greatly increased the composite contractile response to EFS (measured as 'area-under-the curve') and the contractions isolated by L-NAME (respectively, by 22 ± 14% and 334 ± 87%; n = 11/8). After chemoradiotherapy, nitrergic-mediated muscle relaxations occurred more often during EFS (in 29.8 ± 6.1% preparations vs. 12.6 ± 5.1% without chemoradiotherapy, n = 21/18). With L-NAME, the ability of prucalopride to facilitate EFS-evoked contraction was lost and that of donepezil approximately halved (contractions increased by 132 ± 36%; n = 8).

CONCLUSIONS

Several months after chemoradiotherapy, the rectum was not inflamed and myenteric neuron numbers/phenotype unchanged. However, nitrergic activity was increased relative to cholinergic activity, and prokinetic-like drug activity was lost or greatly reduced. Thus, chemoradiotherapy causes long-term changes in neuromuscular functions and markedly reduces the efficacy of drugs for treating constipation.

Tegaserod: What's Old Is New Again

Irritable bowel syndrome with constipation (IBS-C) and chronic idiopathic constipation (CIC) are common gastrointestinal disorders imposing considerable impact on the quality of life and well-being of affected individuals. A paucity of evidence-based treatment options exist for CIC and IBS-C sufferers. Tegaserod, a 5-HT₄ agonist, has a substantial body of preclinical and clinical study evidence to support its beneficial role in modulating sensorimotor function of the luminal gastrointestinal tract. Tegaserod was first approved for use by the U.S. Food and Drug Administration for the management of IBS-C and CIC in 2002 and 2004, respectively. Tegaserod enjoyed a successful uptake in the management of these disorders during its first several years of availability in the United States, but was later withdrawn from the market in 2007 over concerns related to adverse cardiovascular events. Since then, additional safety data has been generated, and following a resubmission and review by the Food and Drug Administration, in April 2019, tegaserod was once again approved for use in IBS-C under a more restricted labeling, confining use to women under 65 years of age without heart disease or additional cardiovascular risk factors. This review summarizes the regulatory journey of tegaserod and details the existing pharmacokinetic, physiologic, clinical, and safety data of tegaserod generated over the last 2 decades. The discussion also examines the future of tegaserod in the treatment of these constipation disorders, as well as its potential role in other related disorders of brain-gut interaction.

Preclinical experimental models for assessing laxative activities of substances/products under investigation: a scoping review of the literature

Constipation is a common gastrointestinal problem worldwide. Its impact on health can range from an unpleasant problem to being seriously troublesome. When lifestyle modification fails to deal with constipation, laxatives are the mainstay of therapy. There are several types of laxatives currently available; however, there still remains a need for better laxatives because certain currently available laxatives are not appropriate for or accessible to some patients. Preclinical experiments to study the laxative potential of substances/products of interest are vital to improving that situation. The selection of appropriate experimental models for assessing the laxative activities of substances/products under investigation is crucial to achieving valid and meaningful results. This article provides a scoping review of the literature, outlining, and summarizing models currently being used in preclinical experiments assessing the laxative activities of substances/products under investigation. The review includes both screening models, e.g., the isolated organ bath system, in vivo fecal assessment and intestinal transit assay, and confirmation models, e.g., in vivo constipation models. Chemical substances/drugs used to induce constipation in in vivo constipation models, e.g., loperamide, diphenoxylate, montmorillonite, and clonidine, as well as standard laxative agents used as a positive control in experimental models, e.g., bisacodyl, carbachol, lactulose, sodium picosulfate, castor oil, phenolphthalein, and yohimbine, are described in detail. The purpose of this article is to assist researchers in the design and implementation of preclinical experimental models for assessing laxative activities of substances/products under investigation to achieve valid and meaningful preclinical results prior to experimentation in humans.

Intraoperative serosal extracellular mapping of the human distal colon: a feasibility study

BACKGROUND

Cyclic motor patterns (CMP) are the predominant motor pattern in the distal colon, and are important in both health and disease. Their origin, mechanism and relation to bioelectrical slow-waves remain incompletely understood. During abdominal surgery, an increase in the CMP occurs in the distal colon. This study aimed to evaluate the feasibility of detecting propagating slow waves and spike waves in the distal human colon through intraoperative, high-resolution (HR), serosal electrical mapping.

METHODS

HR electrical recordings were obtained from the distal colon using validated flexible PCB arrays (6 × 16 electrodes; 4 mm inter-electrode spacing; 2.4 cm2, 0.3 mm diameter) for up to 15 min. Passive unipolar signals were obtained and analysed.

RESULTS

Eleven patients (33-71 years; 6 females) undergoing colorectal surgery under general anaesthesia (4 with epidurals) were recruited. After artefact removal and comprehensive manual and automated analytics, events consistent with regular propagating activity between 2 and 6 cpm were not identified in any patient. Intermittent clusters of spike-like activities lasting 10-180 s with frequencies of each cluster ranging between 24 and 42 cpm, and an average amplitude of 0.54 ± 0.37 mV were recorded.

CONCLUSIONS

Intraoperative colonic serosal mapping in humans is feasible, but unlike in the stomach and small bowel, revealed no regular propagating electrical activity. Although sporadic, synchronous spike-wave events were identifiable. Alternative techniques are required to characterise the mechanisms underlying the hyperactive CMP observed in the intra- and post-operative period.

NEW FINDINGS

The aim of this study was to assess the feasibility of detecting propagating electrical activity that may correlate to the cyclic motor pattern in the distal human colon through intraoperative, high-resolution, serosal electrical mapping. High-resolution electrical mapping of the human colon revealed no regular propagating activity, but does reveal sporadic spike-wave events. These findings indicate that further research into appropriate techniques is required to identify the mechanism of hyperactive cyclic motor pattern observed in the intra- and post-operative period in humans.

The effect of prucalopride on gastric sensorimotor function and satiation in healthy volunteers

BACKGROUND

Gastric motor function alterations have been implicated in the pathogenesis of functional dyspepsia with postprandial distress syndrome (PDS). Prucalopride, a 5-TH4 agonist, is known to stimulate gastrointestinal motility. We aimed to evaluate the effect of prucalopride on gastric sensorimotor function in healthy subjects (HV).

METHODS

Barostat and intragastric pressure (IGP) measurements were performed in 17 HV (59% females, age 29.4 ± 2.7 y) after treatment with placebo or prucalopride (2 mg) (single-blind cross-over). Isobaric stepwise distensions and gastric sensations were assessed to determine gastric compliance and sensitivity. Gastric accommodation (GA) with the barostat was quantified before and after ingestion of 200 ml of a nutrient drink (ND). GA measured by IGP was quantified as the drop of IGP from baseline during the intragastric infusion of ND until maximal satiation (60 ml/min).

KEY RESULTS

Prucalopride did not affect barostat assessed gastric compliance or sensitivity. No differences were observed in GA after prucalopride. During the barostat study, 10 min after the meal, 7 HVs reported significantly higher ratings for nausea after prucalopride (p < 0.001), and vomiting was induced in 4 of the HVs. A positive correlation was observed between the delta mean perception of nausea with the delta mean increase of intra-balloon volume before and after meal ingestion (r = 0.37, p = 0.03). During IGP measurements, no effect on nutrient tolerance was observed and increased cramp severity scores were observed which were associated with a significant increase of distal IGP (r = 0.78, p < 0.0001).

CONCLUSIONS & INFERENCES

Prucalopride does not enhances gastric accommodation but it might increase sensitivity to gastric distention. Furthermore, the increase in sensitivity seems to be related to an increase in nausea with distension. Clinicaltrials.gov: NCT04429802.

The 5-HT4 Receptor Agonist Prucalopride Stimulates Mucosal Growth and Enhances Carbohydrate Absorption in the Ileum of the Mouse

BACKGROUND

Enteric serotonin may function as a mucosal growth factor. Previous work demonstrated increased crypt cell proliferation and intestinal mucosal surface area with potentiation of serotonin. While an indirect mechanism was postulated to explain these effects, the presence of 5-HT4 receptors on enterocytes raises the possibility of a direct action of serotonin. We hypothesized that a 5-HT4 specific agonist, prucalopride, would stimulate intestinal mucosal growth and enhance absorptive function in the murine small intestine.

METHODS

Adult wild-type mice were treated parenterally with prucalopride for 14 days via surgically implanted osmotic pumps. In vivo D-xylose absorption was assessed by oral gavage and serum D-xylose measurements. On day 14, glucose absorption was assessed by instilling a glucose solution into isolated segments of small intestine. The bowel was harvested and examined for morphologic parameters and crypt cell proliferation.

RESULTS

Villus height, crypt depth, and crypt proliferation were significantly increased in the distal small bowel of prucalopride-treated mice compared with control animals. Crypt depth was also increased in the proximal and middle small intestine in treated mice. There was no difference in D-xylose absorption throughout the study period; however, glucose absorption was significantly increased in the distal small intestine of prucalopride-treated mice.

CONCLUSION

Parenteral administration of the 5-HT4 receptor specific agonist, prucalopride, results in morphologic and functional changes in the murine small intestine that are most prominent in the distal small bowel. While further studies are necessary to delineate the mechanism, it is plausible that the effects are mediated by 5-HT4 receptors on enterocytes.

cAMP Catalyzing Phosphodiesterases Control Cholinergic Muscular Activity But Their Inhibition Does Not Enhance 5-HT4 Receptor-Mediated Facilitation of Cholinergic Contractions in the Murine Gastrointestinal Tract

Background: As the signal transduction of 5-HT₄ receptors on cholinergic neurons innervating smooth muscle is controlled by phosphodiesterase (PDE) 4 in porcine stomach and colon, and human large intestine, the in vivo gastroprokinetic effects of a 5-HT₄ receptor agonist might be enhanced by combination with a selective PDE4 inhibitor. The presence of 5-HT₄ receptors on cholinergic neurons towards murine gastrointestinal circular muscle was recently shown. If the control of this receptor pathway by PDE4 is also present in mice, this might be a good model for in vivo testing of the combination therapy. Therefore this study investigates the role of cAMP catalyzing PDEs in smooth muscle cell activity and in the intraneuronal signal transduction of the 5-HT₄ receptors in the gastrointestinal tract of C57Bl/6J mice.

Methods: In circular smooth muscle strips from murine fundus, jejunum, and colon, submaximal cholinergic contractions were induced by either electrical field stimulation (EFS) or by carbachol (muscarinic receptor agonist). The influence of the PDE inhibitors IBMX (non-selective), vinpocetine (PDE1), EHNA (PDE2), cilostamide (PDE3), and rolipram (PDE4) was tested on these contractions and on the facilitating effect of a submaximal concentration of prucalopride (5-HT₄ receptor agonist) on EFS-induced contractions.

Results: In the three gastrointestinal regions, IBMX and cilostamide concentration-dependently decreased carbachol- as well as EFS-induced contractions. Some inhibitory effect was also observed with rolipram. In the fundus a non-significant trend for an enhancement of the facilitating effect of prucalopride on EFS-induced contractions was observed with IBMX, but none of the selective PDE inhibitors enhanced the facilitating effect of prucalopride in fundus, jejunum or colon.

Conclusion: In analogy with the porcine gastrointestinal tract, in murine fundus, jejunum, and colon circular smooth muscle PDE3 is the main regulator of the cAMP turnover, with some contribution of PDE4. In contrast to the porcine gastrointestinal tract, the in vitro facilitation of electrically induced cholinergic contractions by 5-HT₄ receptor stimulation could not be enhanced by specific PDE inhibition. The C57Bl/6J murine model is thus not suitable for in vivo testing of a 5-HT₄ receptor agonist combined with a selective PDE4 inhibitor.

Synergy between 5-HT4 receptor stimulation and phosphodiesterase 4 inhibition in facilitating acetylcholine release in human large intestinal circular muscle

BACKGROUND

Gastroprokinetic properties of 5-HT₄ receptor agonists, such as prucalopride, are attributed to activation of 5-HT₄ receptors on cholinergic nerves innervating smooth muscle in the gastrointestinal smooth muscle layer, increasing acetylcholine release and muscle contraction. In porcine stomach and colon, phosphodiesterase (PDE) 4 has been shown to control the signaling pathway of these 5-HT₄ receptors. The aim of this study was to investigate the PDE-mediated control of these 5-HT₄ receptors in human large intestine.

METHODS

Circular smooth muscle strips were prepared from human large intestine; after incubation with [³H]-choline, electrically induced tritium outflow was determined as a measure for acetylcholine release. The influence of PDE inhibition on the facilitating effect of prucalopride on electrically induced acetylcholine release was studied.

KEY RESULTS

The non-selective PDE inhibitor IBMX enhanced the facilitating effect of prucalopride on electrically induced acetylcholine release. The selective inhibitors vinpocetine (PDE1), EHNA (PDE2) and cilostamide (PDE3) did not influence, while rolipram and roflumilast (PDE4) enhanced the prucalopride-induced facilitation to the same extent as IBMX.

CONCLUSIONS & INFERENCES

In human large intestinal circular muscle, the intracellular pathway of 5-HT₄ receptors facilitating cholinergic neurotransmission to large intestinal circular smooth muscle is controlled by PDE4. If the synergy between 5-HT₄ receptor agonism and PDE4 inhibition is confirmed in a functional assay with electrically induced cholinergic contractions of human large intestinal circular smooth muscle strips, combination of a selective 5-HT₄ receptor agonist with a selective PDE4 inhibitor might enhance the in vivo prokinetic effect of the 5-HT₄ receptor agonist in the large intestine.

Effect of prucalopride on intestinal gas tolerance in patients with functional bowel disorders and constipation

BACKGROUND AND AIM

Patients with functional bowel disorders develop gas retention and symptoms in response to intestinal gas loads that are well tolerated by healthy subjects. Stimulation of 5HT-4 receptors in the gut has both prokinetic and antinociceptive effects. The aim of this study is to determine the effect of prucalopride, a highly selective 5HT-4 agonist, on gas transit and tolerance in women with functional bowel disorders complaining of constipation.

METHODS

Twenty-four women with functional bowel disorders complaining of constipation were included in the study. Patients were studied twice on separate days in a cross-over design. On each study day, an intestinal gas challenge test was performed. During the five previous days, prucalopride (2 mg/day) or placebo was administered. Abdominal symptoms, stool frequency, and stool consistency were recorded during the treatment period on daily questionnaires.

RESULTS

During the gas challenge test, prucalopride did not decrease the volume of gas retained in the subset of patients who had significant gas retention (≥ 200 mL) while on placebo. However, in those patients who had increased symptoms during the gas test (≥ 3 on a 0 to 6 scale) when on placebo, prucalopride did significantly reduce the perception of symptoms (2.3 ± 0.5 mean score vs 3.5 ± 0.3 on placebo; P = 0.045). During the treatment period with prucalopride, patients exhibited an increase in the total number of bowel movements and decreased stool consistency compared with placebo.

CONCLUSION

Prucalopride reduces abdominal symptoms without modifying gas retention when patients with functional bowel disorders are challenged with the gas transit and tolerance test. European Clinical Trials Database (EudraCT2011-006354-86).

5-HT4 receptors facilitate cholinergic neurotransmission throughout the murine gastrointestinal tract

BACKGROUND

In the gastrointestinal tract of several species, facilitating 5-HT₄ receptors were proposed on myenteric cholinergic neurons innervating smooth muscle by in vitro study of the effect of the selective 5-HT₄ receptor agonist prucalopride on submaximal cholinergic contractions. This was not yet established in the murine gastrointestinal tract.

METHODS

In circular smooth muscle strips from murine fundus, jejunum and colon, contractions were induced by electrical field stimulation in the presence of guanethidine, L-NAME and for colon also MRS 2500. Submaximal contractions were induced to study the influence of prucalopride.

KEY RESULTS

Electrical field stimulation at reduced voltage induced reproducible submaximal neurogenic and cholinergic contractions as the contractions were abolished by tetrodotoxin and atropine. Hexamethonium had no systematic inhibitory effect but mecamylamine reduced the responses, suggesting that part of the cholinergic response is due to activation of preganglionic neurons. Prucalopride concentration-dependently increased the submaximal cholinergic contractions in the three tissue types, reaching maximum from 0.03 μmol/L onwards. The facilitation in the different series with 0.03 μmol/L prucalopride ranged from 41% to 104%, 30% to 76% and 24% to 74% in fundus, jejunum, and colon, respectively. The effect of 0.03 μmol/L prucalopride was concentration-dependently inhibited by GR 113808.

CONCLUSIONS & INFERENCES

In the murine gastrointestinal tract, activation of 5-HT₄ receptors with prucalopride enhances cholinergic contractions, illustrating facilitation of myenteric cholinergic neurotransmission. The degree of enhancement with prucalopride is of similar magnitude as previously reported in other species, but the effective concentrations are lower than those needed in the gastrointestinal tract of other species.

Localization of the 5-hydroxytryptamine 4 receptor in equine enteric neurons and extrinsic sensory fibers

BACKGROUND

Serotonin plays a pivotal role in regulating gut motility, visceral sensitivity, and fluid secretion via specific receptors. Among these receptors, 5-HT₄ exerts a prominent control on gut motor function. Although the prokinetic effect exerted by 5-HT₄ agonists is well known, the cellular sites of 5-HT₄ expression remain poorly understood in large mammals, e.g., horses. In this study, we evaluated the distribution of 5-HT₄ in the horse intestine and in foals with enteric aganglionosis, reminiscent of human Hirschsprung's disease.

METHODS

The intestine and spinal ganglia were obtained from three healthy horses and two foals with hereditary ileocolonic aganglionosis. Tissues were processed for immunohistochemistry using a specific antibody to 5-HT₄ and a variety of neuronal markers. Myenteric and submucosal plexus 5-HT₄ -immunoreactive (IR) neurons were quantified as relative percentage (mean±SD) to the total number of neurons counted. Furthermore, the density of 5-HT₄ -IR nerve fibers was evaluated in the mucosa and tunica muscularis.

KEY RESULTS

The 5-HT₄ immunoreactivity was localized to large percentages of myenteric neurons ranging from 28±9% (descending colon) to 63±19% (ileum), and submucosal neurons ranging from 54±6% (ileum) to 68±14% (duodenum). The 5-HT₄ -immunoreactivity was co-expressed by some substance P-IR (SP-IR) spinal ganglion neurons and extrinsic sensory fibers of aganglionic foals.

CONCLUSIONS & INFERENCES

The presence of 5-HT₄ in many enteric and extrinsic sensory neurons and nerve fibers provides solid morphological evidence of the cellular sites of 5-HT₄ expression in horses. The evidence of SP-IR sensory neurons positive for 5-HT₄ suggests its role in visceral sensitivity.

Effects of Hwangryunhaedok-tang on gastrointestinal motility function in mice

AIM

To investigate the effects of Hwangryunhaedok-tang (HHT) on gastrointestinal (GI) motility in mice.

METHODS

The effects of a boiling water extract of HHT (HHTE) on GI motility were investigated by calculating percent intestinal transit rates (ITR%) and gastric emptying (GE) values using Evans Blue and phenol red, respectively, in normal mice and in mice with experimentally induced GI motility dysfunction (GMD). In addition, the effects of the four components of HHT, that is, Gardeniae Fructus (GF), Scutellariae Radix (SR), Coptidis Rhizoma (CR), and Phellodendri Cortex (PC), on GI motility were also investigated.

RESULTS

In normal ICR mice, ITR% and GE values were significantly and dose-dependently increased by the intragastric administration of HHTE (0.1-1 g/kg). The ITR% values of GMD mice were significantly lower than those of normal mice, and these reductions were significantly and dose-dependently inhibited by HHTE (0.1-1 g/kg). Additionally, GF, CR, and PC dose-dependently increased ITR% and GE values in normal and GMD mice.

CONCLUSION

These results suggest that HHT is a novel candidate for the development of a gastroprokinetic agent for the GI tract.

The 5-HT4 receptor agonist prucalopride does not facilitate cholinergic neurotransmission in circular and longitudinal smooth muscle preparations of equine mid-jejunum

BACKGROUND

Postoperative ileus (POI) remains an important cause of death in horses. The recently developed selective 5-HT₄ receptor agonists such as prucalopride target 5-HT₄ receptors on myenteric cholinergic neurons to enhance acetylcholine release and GI motility. No clearcut in vitro evaluation whether highly selective 5-HT₄ receptor agonists enhance submaximal cholinergic neurotransmission towards the muscle layer has been performed in horses.

OBJECTIVES

To identify functional 5-HT₄ receptors in equine jejunum.

STUDY DESIGN

In vitro experimental study.

METHODS

Circular and longitudinal smooth muscle strips (mid-jejunum) were mounted in organ baths between 2 platinum electrodes allowing electrical field stimulation (EFS). To delineate the conditions to obtain purely cholinergic responses, voltage-response curves were studied. To investigate the influence of prucalopride and 5-HT, submaximal cholinergic contractions at a single voltage were induced.

RESULTS

In circular and longitudinal strips, EFS induced voltage-dependent neurogenic on-contractions when the bathing medium contained a NO-synthesis inhibitor and apamin to prevent inhibitory responses to NO and ATP. Contractions at a voltage inducing 50% of maximal amplitude were cholinergic, as they were blocked by atropine. These contractions were not influenced by prucalopride (up to 3μM), even in the presence of the phosphodiesterase inhibitor isobutyl-methyl-xanthine to inhibit breakdown of the second messenger of 5-HT₄ receptors, cAMP. Also the full 5-HT₄ receptor agonist 5-HT did not influence the EFS-induced submaximal cholinergic contractions. Moreover, prucalopride did not influence muscle tone continuously enhanced with KCl.

CONCLUSIONS

There are no functional 5-HT₄ receptors on myenteric cholinergic neurons nor muscular 5-HT₄ receptors in equine jejunum.

Prokinetic effects of a new 5-HT4 agonist, YKP10811, on gastric motility in dogs

BACKGROUND AND AIM

Prokinetics have been considered the first-line medicine for treating delayed gastric emptying. The aim of this study was to explore the effects and mechanisms of a new 5-HT₄ receptor agonist, YKP10811, on gastric motility in dogs.

METHODS

Four experiments were performed in dogs: (i) dose-response effects of YKP10811 on liquid gastric emptying; (ii) effects and mechanisms of YKP10811 on solid gastric emptying delayed by glucagon; (iii) effects of low-dose YKP10811 on antral contractions; and (iv) effects of low-dose YKP10811 on gastric accommodation.

RESULTS

No adverse events or cardiac dysrhythmia was noted. (i) High-dose YKP10811 (30 mg/kg) accelerated liquid gastric emptying from 15 to 90 min without inducing adverse events or cardiac dysrhythmia. YKP10811 at low doses (0.3, 1, and 3 mg/kg) accelerated gastric emptying in a dose-dependent manner. (ii) YKP10811 (0.1 mg/kg), but not tegaserod (0.3 mg/kg), significantly accelerated glucagon-induced delayed gastric emptying of solid, and the effect was completely blocked by GR113808. (iii) YKP10811 (0.3 mg/kg) enhanced antral contractions. (iv) YKP10811 did not alter gastric accommodation.

CONCLUSIONS

YKP10811 seems to improve antral contractions and accelerate gastric emptying without altering gastric accommodation in dogs via the 5-HT₄ mechanism and is substantially more potent than tegaserod. No adverse events were noted at a dose 300 times the lowest effective dose. YKP10811 may have a therapeutic potential for gastroparesis.

[Current Status of Translational Research on Constipation]

Constipation is one of the most common gastrointestinal disorders with a prevalence up to 16.5% in the general population. It is frequently multifactorial and the pathophysiologic mechanism of constipation is not fully understood. Many preclinical studies of constipation have used animal models. Translational research using these animal models is essential to the investigation of neurogenic and myogenic mechanisms of colon, and to the estimation of the clinical efficacy of new drugs. In this review, we discuss some of the current translational research projects on constipation using animal models.

Redundant Gs-coupled serotonin receptors regulate amyloid-β metabolism in vivo

BACKGROUND

The aggregation of amyloid-β (Aβ) into insoluble plaques is a hallmark pathology of Alzheimer's disease (AD). Previous work has shown increasing serotonin levels with selective serotonin re-uptake inhibitor (SSRI) compounds reduces Aβ in the brain interstitial fluid (ISF) in a mouse model of AD and in the cerebrospinal fluid of humans. We investigated which serotonin receptor (5-HTR) subtypes and downstream effectors were responsible for this reduction.

RESULTS

Agonists of 5-HT4R, 5-HT6R, and 5-HT7R significantly reduced ISF Aβ, but agonists of other receptor subtypes did not. Additionally, inhibition of Protein Kinase A (PKA) blocked the effects of citalopram, an SSRI, on ISF Aβ levels. Serotonin signaling does not appear to change gene expression to reduce Aβ levels in acute timeframes, but likely acts within the cytoplasm to increase α-secretase enzymatic activity. Broad pharmacological inhibition of putative α-secretases increased ISF Aβ and blocked the effects of citalopram.

CONCLUSIONS

In total, these studies map the major signaling components linking serotonin receptors to suppression of brain ISF Aβ. These results suggest the reduction in ISF Aβ is mediated by a select group of 5-HTRs and open future avenues for targeted therapy of AD.

The effect of levosulpiride on in vitro motor patterns in the human gastric fundus, antrum, and jejunum

BACKGROUND

Levosulpiride is a 5HT4 agonist/D2 antagonist prokinetic agent used to improve gastric emptying in patients with functional dyspepsia or gastroparesis. The aim of this study was to characterize its effect on the main in vitro motility patterns in the human fundus, antrum, and jejunum.

METHODS

Circular muscle strips from human stomach (antrum and fundus) and jejunum, obtained from 46 patients undergoing bariatric surgery, were studied using organ baths. Enteric motor neurons (EMNs) were stimulated by electrical field stimulation (EFS).

KEY RESULTS

Levosulpiride, caused an increase in the EFS-induced cholinergic contractions in the gastric antrum (+37 ± 15.18% at 100 μM, pEC50 = 4.46 ± 0.14; p < 0.05, n = 8) and jejunum (+45.4 ± 22.03% at 100 μM, pEC50 = 3.78 ± 6.81; p < 0.05, n = 5), but not in the gastric fundus. It also caused a slight decrease in tone and frequency of spontaneous contractions in the jejunum, but did not have any major effect on tone or spontaneous contractions in the stomach. It did not have any effect on EFS-induced relaxations mediated by nitric oxide (NO) in the stomach (antrum and fundus) and by NO and ATP in the jejunum.

CONCLUSIONS & INFERENCES

Our results suggest that the prokinetic effects of levosulpiride in humans are mainly due to the facilitation of the release of acetylcholine by enteric motor neurons in the gastric antrum and the jejunum.

Synergistic effect between 5-HT4 receptor agonist and phosphodiesterase 4-inhibitor in releasing acetylcholine in pig gastric circular muscle in vitro

5-HT4 receptor agonists have a gastroprokinetic effect by facilitating acetylcholine release from cholinergic nerves innervating gastrointestinal smooth muscle. The role of phosphodiesterase (PDE) 4 in the signal transduction pathway of the 5-HT4 receptors located on the cholinergic neurons towards the circular muscle layer in pig stomach was investigated by analysis of acetylcholine release. Circular muscle strips were prepared from pig proximal stomach and tritium outflow, induced by electrical field stimulation, was studied as a marker for acetylcholine release after incubation with [(3)H]-choline. The PDE4-inhibitor roflumilast concentration-dependently (0.1-1µM) enhanced the facilitating effect of a submaximally effective concentration of the 5-HT4 receptor agonist prucalopride (0.01µM) on electrically induced acetylcholine release. Roflumilast (0.3µM) enhanced acetylcholine release per se but in the combined presence of roflumilast and prucalopride, acetylcholine release was enhanced more than the sum of the effect of the 2 compounds alone. The 5-HT4 receptor agonist velusetrag concentration-dependently (0.01-0.1µM) enhanced acetylcholine release; the effect of the minimally effective concentration (0.01µM) was significantly enhanced by 1µM of the PDE4-inhibitor rolipram, again to a level higher than the sum of the effect of the 2 compounds alone. The synergistic effect between 5-HT4 receptor agonists and PDE4-inhibitors demonstrates that the intracellular pathway of the 5-HT4 receptors located on cholinergic neurons towards pig gastric circular muscle is controlled by PDE4. Combining a 5-HT4 receptor agonist with a PDE4-inhibitor might thus enhance its gastroprokinetic effect.

Distribution of 5-HT3, 5-HT4, and 5-HT7 Receptors Along the Human Colon

Background/Aims

Several disorders of the gastrointestinal tract are associated with abnormal serotonin (5-HT) signaling or metabolism where the 5-HT₃ and 5-HT₄ receptors are clinically relevant. The aim was to examine the distribution of 5-HT₃, 5-HT₄, and 5-HT₇ receptors in the normal human colon and how this is associated with receptor interacting chaperone 3, G protein coupled receptor kinases, and protein LIN-7 homologs to extend previous observations limited to the sigmoid colon or the upper intestine.

Methods

Samples from ascending, transverse, descending, and sigmoid human colon were dissected into 3 separate layers (mucosa, longitudinal, and circular muscles) and ileum samples were dissected into mucosa and muscle layers (n = 20). Complementary DNA was synthesized by reverse transcription from extracted RNA and expression was determined by quantitative or end point polymerase chain reaction.

Results

The 5-HT₃ receptor subunits were found in all tissues throughout the colon and ileum. The A subunit was detected in all samples and the C subunit was expressed at similar levels while the B subunit was expressed at lower levels and less frequently. The 5-HT₃ receptor E subunit was mainly found in the mucosa layers. All splice variants of the 5-HT₄ and 5-HT₇ receptors were expressed throughout the colon although the 5-HT₄ receptor d, g, and i variants were expressed less often.

Conclusions

The major differences in 5-HT receptor distribution within the human colon are in relation to the mucosa and muscular tissue layers where the 5-HT₃ receptor E subunit is predominantly found in the mucosal layer which may be of therapeutic relevance.

Efficacy and safety of prucalopride in adults and children with chronic constipation

INTRODUCTION

Chronic constipation (CC) is a debilitating condition with high prevalence rates both in children and adults. Despite the broad range of medical and pharmaceutical treatments, the bowel function does not restore in a fair amount of patients. Prucalopride is a first-in-class selective, high affinity serotonin 5-hydroxytryptamine type 4 (5-HT4) receptor agonist promoting gastro-intestinal prokinetic activity and has been evaluated for the treatment of CC.

AREAS COVERED

A PubMed search (1965 - 2014) using the following terms alone or in combination: prucalopride, 5-HT4, R093877, safety, toxicity, pharmacokinetics, pharmacodynamics, transit, cardiac, human ether-a-go-go related gene (hERG), arrhythmia, potassium current, elderly, children.

EXPERT OPINION

Prucalopride, a highly selective 5-HT4 receptor agonist, stimulates gastrointestinal motility and has been proven to be effective in the treatment of CC in adults by increasing stool frequency, reducing constipation-related symptoms and improving quality of life (QoL). The safety and tolerability have been proven to be excellent. More research would be preferable on the effect of prucalopride on men, children and in other gastrointestinal motility disorders.

Cholinergic interactions between donepezil and prucalopride in human colon: potential to treat severe intestinal dysmotility

BACKGROUND AND PURPOSE

Cholinesterase inhibitors such as neostigmine are used for acute colonic pseudo-obstruction, but cardio-bronchial side-effects limit use. To minimize side-effects, lower doses could be combined with a 5-HT4 receptor agonist, which also facilitates intestinal cholinergic activity. However, safety concerns, especially in the elderly, require drugs with good selectivity of action. These include the AChE inhibitor donepezil (used for Alzheimer's disease, with reduced cardio-bronchial liability) and prucalopride, the first selective, clinically available 5-HT4 receptor agonist. This study examined their individual and potential synergistic activities in human colon.

EXPERIMENTAL APPROACH

Neuronally mediated muscle contractions and relaxations of human colon were evoked by electrical field stimulation (EFS) and defined phenotypically as cholinergic, nitrergic or tachykinergic using pharmacological tools; the effects of drugs were determined as changes in 'area under the curve'.

KEY RESULTS

Prucalopride increased cholinergically mediated contractions (EC50 855 nM; 33% maximum increase), consistent with its ability to stimulate intestinal motility; donepezil (477%) and neostigmine (2326%) had greater efficacy. Concentrations of donepezil (30-100 nM) found in venous plasma after therapeutic doses had minimal ability to enhance cholinergic activity. However, donepezil (30 nM) together with prucalopride (3, 10 μM) markedly increased EFS-evoked contractions compared with prucalopride alone (P = 0.04). For example, the increases observed with donepezil and prucalopride 10 μM together or alone were, respectively, 105 ± 35%, 4 ± 6% and 35 ± 21% (n = 3-7, each concentration).

CONCLUSIONS AND IMPLICATIONS

Potential synergy between prucalopride and donepezil activity calls for exploration of this combination as a safer, more effective treatment of colonic pseudo-obstruction.

Regionally dependent neuromuscular functions of motilin and 5-HT₄ receptors in human isolated esophageal body and gastric fundus

BACKGROUND

Motilin agonists promote human gastric motility and cholinergic activity, but excitatory and inhibitory actions are reported in the esophagus. The effect of 5-HT₄ agonists in esophagus is also unclear. Perhaps the use of drugs with additional actions explains the variation. The aim, therefore, was to examine how motilin and prucalopride, selective motilin and 5-HT₄ receptor agonists, modulate neuromuscular functions in human esophagus and gastric fundus.

METHODS

Electrical field stimulation (EFS) evoked nerve-mediated contractions of circular and longitudinal muscle from human esophageal body and circular muscle from gastric fundus.

KEY RESULTS

In esophageal circular muscle EFS evoked brief contraction, followed by another contraction on termination of EFS, each prevented by atropine. Nitric oxide synthase inhibition facilitated contraction during EFS and the overall contraction became monophasic. In esophagus longitudinal muscle and gastric fundus, EFS evoked cholinergically mediated, monophasic contractions, attenuated by simultaneous nitrergic activation. Motilin (100-300 nM) reduced esophagus circular muscle contractions during EFS, unaffected by L-NAME or apamin. Motilin 300 nM also reduced EFS-evoked contractions of longitudinal muscle. Similar concentrations of motilin facilitated cholinergic activity in the fundus and increased baseline muscle tension. Prucalopride facilitated EFS-evoked contractions in esophagus (tested at 30 μM) and fundus (0.1-30 μM).

CONCLUSIONS & INFERENCES

Selective motilin and 5-HT₄ agonists have different, region-dependent abilities to modulate human esophageal and stomach neuromuscular activity, exemplified by weak inhibition (motilin) or excitation (5-HT₄) in esophageal body and excitation for both in stomach. In different patients with motility dysfunctions, motilin and 5-HT₄ agonists may reduce gastro-esophageal reflux in different ways.

Prucalopride: a review of its use in the management of chronic constipation

The highly selective serotonin 5-HT4 receptor agonist prucalopride (Resolor(®), Resotran(®), Resotrans(®)) is indicated for the treatment of chronic constipation. In four randomized, double-blind, multicentre, 12-week trials in patients (predominantly women) with chronic constipation, oral prucalopride 2 mg once daily improved bowel function to a significantly greater extent than placebo, with a significantly greater proportion of prucalopride than placebo recipients achieving an average of ≥3 spontaneous, complete bowel movements per week (primary endpoint). Significantly greater improvements in health-related quality of life, patient satisfaction with treatment and bowel habit, and a range of constipation-related symptoms were also seen with prucalopride than with placebo. Satisfaction with treatment and bowel habit was maintained with prucalopride in the longer term. Prucalopride was generally well tolerated in patients with chronic constipation, with the most commonly reported adverse events (headache, nausea, abdominal pain, diarrhoea) primarily occurring on the first day of treatment. During the clinical trials, no cardiovascular safety issues have arisen in patients with chronic constipation receiving prucalopride. In conclusion, prucalopride is an important option for use in patients with chronic constipation who have not experienced adequate relief with laxatives.

Preclinical gastrointestinal prokinetic efficacy and endocrine effects of the ghrelin mimetic RM-131

AIMS

The 28 amino acid hormone ghrelin, the natural ligand for the growth hormone secretagogue, or ghrelin receptor (GHR), has diverse physiological functions, including a possible role as a gastrointestinal prokinetic. The synthetic ghrelin mimetic RM-131 is in Phase II clinical trials for treatment of diabetic gastroparesis and other gastrointestinal (GI) disorders. We aimed to determine the relative potency of RM-131, when compared to other GI ghrelin mimetics, to predict efficacy and determine the role of RM-131 in models of inflammatory bowel disease.

MAIN METHODS

We evaluated and compared ghrelin, RM-131 and other synthetic ghrelin mimetics for their prokinetic potency in models of gastrointestinal disorders in the rat and we evaluated the endocrine (rats and dogs) and anti-inflammatory effects (mice) of the ghrelin mimetic RM-131.

KEY FINDINGS

The pentapeptide RM-131 increased gastric emptying in rodent models of ileus. RM-131 is about 100-fold more potent than human ghrelin and is 600 to 1800-fold more potent, when compared to several investigational ghrelin mimetics tested in clinical trials. RM-131 has anti-inflammatory effects and significantly increases survival and reduces macroscopic markers of tissue damage in a TNBS model of inflammatory bowel disease. RM-131 treatment shows a transient increase in growth hormone levels in Beagle dogs and rats, returning to baseline upon chronic treatment. Significant effects on glucose and insulin are not observed in chronic studies.

SIGNIFICANCE

RM-131's potency, efficacy and endocrine profile, are promising attributes for the treatment of diverse functional gastrointestinal disorders in humans.

Pharmacological profile of DA-6886, a novel 5-HT4 receptor agonist to accelerate colonic motor activity in mice

DA-6886, the gastrointestinal prokinetic benzamide derivative is a novel 5-HT4 receptor agonist being developed for the treatment of constipation-predominant irritable bowel syndrome (IBS-C). The purpose of this study was to characterize in vitro and in vivo pharmacological profile of DA-6886. We used various receptor binding assay, cAMP accumulation assay, organ bath experiment and colonic transit assay in normal and chemically constipated mice. DA-6886 exhibited high affinity and selectivity to human 5-HT4 receptor splice variants, with mean pKi of 7.1, 7.5, 7.9 for the human 5-HT4a, 5-HT4b and 5-HT4d, respectively. By contrast, DA-6886 did not show significant affinity for several receptors including dopamine D2 receptor, other 5-HT receptors except for 5-HT2B receptor (pKi value of 6.2). The affinity for 5-HT4 receptor was translated into functional agonist activity in Cos-7 cells expressing 5-HT4 receptor splice variants. Furthermore, DA-6886 induced relaxation of the rat oesophagus preparation (pEC50 value of 7.4) in a 5-HT4 receptor antagonist-sensitive manner. The evaluation of DA-6886 in CHO cells expressing hERG channels revealed that it inhibited hERG channel current with an pIC50 value of 4.3, indicating that the compound was 1000-fold more selective for the 5-HT4 receptor over hERG channels. In the normal ICR mice, oral administration of DA-6886 (0.4 and 2mg/kg) resulted in marked stimulation of colonic transit. Furthermore, in the loperamide-induced constipation mouse model, 2mg/kg of DA-6886 significantly improved the delay of colonic transit, similar to 10mg/kg of tegaserod. Taken together, DA-6886 is a highly potent and selective 5-HT4 receptor agonist to accelerate colonic transit in mice, which might be therapeutic agent having a favorable safety profile in the treatment of gastrointestinal motor disorders such as IBS-C and chronic constipation.

Serotonin mediation of early memory formation via 5-HT2B receptor-induced glycogenolysis in the day-old chick

Investigation of the effects of serotonin on memory formation in the chick revealed an action on at least two 5-HT receptors. Serotonin injected intracerebrally produced a biphasic effect on memory consolidation with enhancement at low doses and inhibition at higher doses. The non-selective 5-HT receptor antagonist methiothepin and the selective 5-HT2B/C receptor antagonist SB221284 both inhibited memory, suggesting actions of serotonin on at least two different receptor subtypes. The 5-HT2B/C and astrocyte-specific 5-HT receptor agonist, fluoxetine and paroxetine, enhanced memory and the effect was attributed to glycogenolysis. Inhibition of glycogenolysis with a low dose of DAB (1,4-dideoxy-1,4-imino-D-arabinitol) prevented both serotonin and fluoxetine from enhancing memory during short-term memory but not during intermediate memory. The role of serotonin on the 5-HT2B/C receptor appears to involve glycogen breakdown in astrocytes during short-term memory, whereas other published evidence attributes the second period of glycogenolysis to noradrenaline.

Translational neuropharmacology: the use of human isolated gastrointestinal tissues

Translational sciences increasingly emphasize the measurement of functions in native human tissues. However, such studies must confront variations in patient age, gender, genetic background and disease. Here, these are discussed with reference to neuromuscular and neurosecretory functions of the human gastrointestinal (GI) tract. Tissues are obtained after informed consent, in collaboration with surgeons (surgical techniques help minimize variables) and pathologists. Given the difficulties of directly recording from human myenteric neurones (embedded between muscle layers), enteric motor nerve functions are studied by measuring muscle contractions/relaxations evoked by electrical stimulation of intrinsic nerves; responses are regionally dependent, often involving cholinergic and nitrergic phenotypes. Enteric sensory functions can be studied by evoking the peristaltic reflex, involving enteric sensory and motor nerves, but this has rarely been achieved. As submucosal neurones are more accessible (after removing the mucosa), direct neuronal recordings are possible. Neurosecretory functions are studied by measuring changes in short-circuit current across the mucosa. For all experiments, basic questions must be addressed. Because tissues are from patients, what are the controls and the influence of disease? How long does it take before function fully recovers? What is the impact of age- and gender-related differences? What is the optimal sample size? Addressing these and other questions minimizes variability and raises the scientific credibility of human tissue research. Such studies also reduce animal use. Further, the many differences between animal and human GI functions also means that human tissue research must question the ethical validity of using strains of animals with unproved translational significance.

Effects of 5-HT2B, 5-HT3 and 5-HT4 receptor antagonists on gastrointestinal motor activity in dogs

AIM: To study the effects of 5-hydroxytryptamine (5-HT) receptor antagonists on normal colonic motor activity in conscious dogs.

METHODS: Colonic motor activity was recorded using a strain gauge force transducer in 5 dogs before and after 5-HT2B, 5-HT3 and 5-HT4 receptor antagonist administration. The force transducers were implanted on the serosal surfaces of the gastric antrum, terminal ileum, ileocecal sphincter and colon. Test materials or vehicle alone was administered as an intravenous bolus injection during a quiescent period of the whole colon in the interdigestive state. The effects of these receptor antagonists on normal gastrointestinal motor activity were analyzed.

RESULTS: 5-HT2B, 5-HT3 and 5-HT4 receptor antagonists had no contractile effect on the fasting canine terminal ileum. The 5-HT3 and 5-HT4 receptor antagonists inhibited phase III of the interdigestive motor complex of the antrum and significantly inhibited colonic motor activity. In the proximal colon, the inhibitory effect was dose dependent. Dose dependency, however, was not observed in the distal colon. The 5-HT2B receptor antagonist had no contractile effect on normal colonic motor activity.

CONCLUSION: The 5-HT3 and 5-HT4 receptor antagonists inhibited normal colonic motor activity. The 5-HT2B receptor antagonist had no contractile effect on normal colonic motor activity.

Predominant mucosal expression of 5-HT4(+h) receptor splice variants in pig stomach and colon

AIM: To investigate cellular 5-HT_4(-h/+h) receptor distribution, particularly in the epithelial layer, by laser microdissection and polymerase chain reaction (PCR) in porcine gastrointestinal (GI) tissues.

METHODS: A stepwise approach was used to evaluate RNA quality and to study cell-specific 5-HT₄ receptor mRNA expression in the porcine gastric fundus and colon descendens. After freezing, staining and laser microdissection and pressure catapulting (LMPC), RNA quality was evaluated by the Experion automated electrophoresis system. 5-HT₄ receptor and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) expressions were examined by endpoint reverse transcription (RT)-PCR in mucosal and muscle-myenteric plexus (MMP) tissue fractions, in mucosal and MMP parts of hematoxylin and eosin (HE) stained tissue sections and in microdissected patches of the epithelial and circular smooth muscle cell layer in these sections. Pig gastric fundus tissue sections were also stained immunohistochemically (IHC) for enterochromaffin cells (EC cells; MAB352); these cells were isolated by LMPC and examined by endpoint RT-PCR.

RESULTS: After HE staining, the epithelial and circular smooth muscle cell layer of pig colon descendens and the epithelial cell layer of gastric fundus were identified morphologically and isolated by LMPC. EC cells of pig gastric fundus were successfully stained by IHC and isolated by LMPC. Freezing, HE and IHC staining, and LMPC had no influence on RNA quality. 5-HT₄ receptor and GAPDH mRNA expressions were detected in mucosa and MMP tissue fractions, and in mucosal and MMP parts of HE stained tissue sections of pig colon descendens and gastric fundus. In the mucosa tissue fractions of both GI regions, the expression of h-exon containing receptor [5-HT_4(+h) receptor] mRNA was significantly higher (P < 0.01) compared to 5-HT_4(-h) receptor expression, and a similar trend was obtained in the mucosal part of HE stained tissue sections. Large microdissected patches of the epithelial and circular smooth muscle cell layer of pig colon descendens and of the epithelial cell layer of pig gastric fundus, also showed 5-HT₄ receptor and GAPDH mRNA expression. No 5-HT₄ receptor mRNA expression was detected in gastric LMPC-isolated EC cells from IHC stained tissues, which cells were positive for GAPDH.

CONCLUSION: Porcine GI mucosa predominantly expresses 5-HT_4(+h) receptor splice variants, suggesting their contribution to the 5-HT₄ receptor-mediated mucosal effects of 5-HT.

Prucalopride: evaluation of the pharmacokinetics, pharmacodynamics, efficacy and safety in the treatment of chronic constipation

INTRODUCTION

Prucalopride is the first member of a novel class of 5-HT(4) receptor agonist which has been extensively evaluated for the treatment of chronic constipation. Predominantly, prucalopride is currently used to treat patients that show an insufficient response to laxatives as an alternative form of therapy.

AREAS COVERED

The following article provides the reader with a systematic review of the literature on prucalopride. Specifically, the article reviews the currently literature on the pharmacokinetics and the pharmacodynamics of the drugs as well as reviewing literature on its efficacy. Furthermore, the authors also highlight the safety and tolerability of the drug that have been demonstrated in its clinical development.

EXPERT OPINION

Prucalopride is an important addition to the therapeutic abilities for treating chronic constipation, especially in females poorly responding to laxatives. The safety profile of the drug, to date, is favorable. There is also the possibility that prucalopride might be of benefit to other disorders of gastrointestinal motility with a number of studies currently in progress, which are evaluating alternative applications.

Anti-diarrheal activity and toxicity of Learng Pid Samud recipe

Learng Pid Samud (LPS) recipe is a traditional remedy in Thai folk medicine to ease the common diarrhea. The anti-diarrheal potential of LPS recipe was herein examined in vitro using a guinea-pig ileum model. The LPS exerted an inhibitory effect on acetylcholine-induced smooth muscle contraction in the guinea pig ileum. Significantly, not only did the LPS reduce the total amount of feces in the induced diarrhea rats, but also the intestinal transit in the charcoal meal test. A single oral administration with the recipe at 5,000 mg/kg did not cause acute toxicity and the daily oral administration (1,000, 2,000 and 4,000 mg/kg) for 90 days in rats did not produce any toxic signs and symptoms. In conclusion, the Learng Pid Samud recipe remedy is evidently safe and effective for the anti-diarrheal treatment which supports its therapeutic uses in the alternative medicine.

The facilitating effect of prucalopride on cholinergic neurotransmission in pig gastric circular muscle is regulated by phosphodiesterase 4

The influence of the selective 5-HT(4) receptor agonist prucalopride on acetylcholine release from cholinergic nerve endings innervating pig gastric circular muscle and the possible regulation of this effect by phosphodiesterases (PDEs) was investigated, as PDEs have been shown to control the response to 5-HT(4) receptor activation in pig left atrium. Circular muscle strips were prepared from pig proximal stomach and either submaximal cholinergic contractions or tritium outflow after incubation with [(3)H]-choline, induced by electrical field stimulation, were studied. Prucalopride concentration-dependently increased the amplitude of submaximal cholinergic contractions and of acetylcholine release induced by electrical field stimulation. The effect of the highest concentration tested (0.3 μM) on cholinergic contractions was antagonized by the selective 5-HT(4) receptor antagonist GR113808 but not by granisetron or methysergide; the antagonism of prucalopride by GR113808 was confirmed in the release assay. The non-selective PDE-inhibitor 3-isobutyl-methyl-xanthine (IBMX) concentration-dependently reduced the amplitude of the cholinergic contractions; 3 μM IBMX reduced the cholinergic contractions maximally by 16% but it enhanced the facilitating effect of prucalopride from 51 to 83%. IBMX (10 μM) induced and enhanced the facilitating effect of prucalopride on electrically induced acetylcholine release. The selective inhibitors vinpocetine (PDE1), EHNA (PDE2) and cilostamide (PDE3) did not influence the effect of prucalopride on acetylcholine release but the PDE4-inhibitor rolipram (1 μM) enhanced the facilitating effect of prucalopride to the same extent as IBMX. These results demonstrate that 5-HT(4) receptors are present on the cholinergic nerves towards the pig gastric circular muscle, facilitating acetylcholine release; the intracellular transduction pathway of this facilitation is regulated by PDE4. Combination of a 5-HT(4) receptor agonist with selective inhibition of the PDE involved in this regulation of transmitter release might enhance the prokinetic effect of the 5-HT(4) receptor agonist.

NUMERICAL SIMULATION OF THE ROLE OF CO-TRANSMISSION BY ACETYLCHOLINE AND SEROTONIN ON MOTILITY OF THE GUT

Electrophysiological mechanisms of co-transmission by serotonin (5-HT) and acetylcholine (ACh), co-expression of their receptor types, i.e., 5-HT type 3 and 4, nicotinic cholinerginc (nACh) and muscarinic cholinergic (μACh), and effects of selective and non-selective 5-HT3 and 5-HT4 receptor agonists/antagonists, on electromechanical activity of the gut were studied numerically. Two series of numerical experiments were performed. First, the dynamics of the generation and propagation of electrical signals interconnected with the primary sensory (AH) neurons, motor (S) neurons and smooth muscle cells were studied in a one-dimensional model. Simulations showed that stimulation of the 5-HT3 receptors reduced the threshold of activation of the mechanoreceptors by 17.6%. Conjoint excitation of the 5-HT3 and 5-HT4 receptors by endogenous serotonin converted the regular firing pattern of electrical discharges of the AH and S neurons to a beating mode. Activation confined to 5-HT3 receptors, located on the somas of the adjacent AH and S type neurons, could not sustain normal signal transduction between them. It required ACh as a co-transmitter and co-activation of the nACh receptors. Application of selective 5-HT3 receptor antagonists inhibited dose-dependently the production of action potentials at the level of mechanoreceptors and the soma of the primary sensory neuron and increased the threshold activation of the mechanoreceptors. Normal mechanical contractile activity depended on co-stimulation of the 5-HT4 and μACh receptors on the membrane of smooth muscle cells. In the second series of simulations, which involved a spatio-temporal model of the functional unit, effects of co-transmission by ACh and 5-HT on the electromechanical response in a segment of the gut were analyzed. Results indicated that propagation of the wave of excitation between the AH and S neurons within the myenteric nervous plexus in the presence of 5-HT3 receptor antagonists was supported by co-release of ACh. Co-stimulation of 5-HT3, nACh and μACh receptors impaired propulsive activity of the gut. The bolus showed uncoordinated movements. In an ACh-free environment Lotronex (GlaxoSmithKline), a 5-HT3 receptor antagonist, significantly increased the transit time of the pellet along the gut. In the presence of ACh, Lotronex produced intensive tonic-type contractions in the longitudinal and circular smooth muscle layers and eliminated propulsive activity. The 5HT4 receptor agonist, Zelnorm (Novartis), preserved the reciprocal electromechanical relationships between the longitudinal and circular smooth muscle layers. The drug changed the normal propulsive pattern of activity to an expulsive (non-mixing) type. Treatment of the gut with selective 5HT4 receptor antagonists increased the transit time by disrupting the migrating myoelectrical complex. Cisapride (Janssen), a mixed 5HT3 and 5HT4 receptor agonist, increased excitability of the AH and S neurons and the frequency of slow waves. Longitudinal and circular smooth muscle syncytia responded with the generation of long-lasting tonic contractions, resulting in a "squeezing" type of pellet movement. Comparison of the theoretical results obtained on one-dimensional and spatio-temporal models to in vivo and in vitro experimental data indicated satisfactory qualitative, and where available, quantitative agreement.

Effect of jatrorrhizine on delayed gastrointestinal transit in rat postoperative ileus

OBJECTIVES

Postoperative ileus is major cause of postoperative complication and prolonged hospitalization. Jatrorrhizine, which is a protoberberine alkaloid isolated from the medicinal plants Berberis aristata and Coptis chinensis, has been found to increase contractility of gastric antral and ileum smooth muscles of rat gastrointestinal tract. We have investigated whether jatrorrhizine could offset gastrointestinal transit in rat with postoperative ileus.

METHODS

Postoperative ileus was induced by laparotomy with intestinal manipulation under anaesthesia. Gastrointestinal transit was evaluated by measurement of gastric emptying, geometric centre and the migration of Evans blue.

KEY FINDINGS

Postoperative ileus significantly delayed gastric emptying and intestinal transit. Jatrorrhizine dose-dependently (0.1, 0.3 and 1 mg/kg) offset delayed gastric emptying and intestinal transit (geometric centre and the migration of Evans blue) in postoperative ileus. Pretreatment of animals with atropine inhibited the action of jatrorrhizine on gastric emptying and intestinal transit, but pretreatment of animals with SB204070 did not influence the effect of jatrorrhizine on gastric emptying and intestinal transit in postoperative ileus.

CONCLUSIONS

Jatrorrhizine offset postoperative ileus-induced delayed gastric emptying and intestinal transit in rats, an action mediated via the cholinergic pathway, but not involving activation of 5-HT(4) receptors.

Investigation of neurogenic excitatory and inhibitory motor responses and their control by 5-HT(4) receptors in circular smooth muscle of pig descending colon

The aim of this study was to investigate whether the pig colon descendens might be a good model for the responses mediated via the different locations of human colonic 5-HT(4) receptors. The intrinsic excitatory and inhibitory motor neurotransmission in pig colon descendens was therefore first characterized. In circular smooth muscle strips, electrical field stimulation (EFS) at basal tone induced only in the combined presence of the NO synthase inhibitor N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME) and the SK channel blocker apamin voltage-dependent on-contractions. These on-contractions were largely reduced by the neuronal conductance blocker tetrodotoxin (TTX) and by the muscarinic receptor antagonist atropine, illustrating activation of cholinergic neurons. The 5-HT(4) receptor agonist prucalopride facilitated submaximal EFS-evoked cholinergic contractions and this effect was prevented by the 5-HT(4) receptor antagonist GR113808, supporting the presence of facilitating 5-HT(4) receptors on the cholinergic nerve endings innervating circular muscle in pig colon descendens. Relaxations were induced by EFS in strips pre-contracted with substance P in the presence of atropine. The responses at lower stimulation voltages were abolished by TTX. L-NAME or apamin alone did not influence or only moderately reduced the relaxations, but L-NAME plus apamin abolished the relaxations at lower stimulation voltages, suggesting that NO and ATP act as inhibitory neurotransmitters in a redundant way. Prucalopride did not influence the EFS-induced relaxations at lower stimulation voltage, nor did it per se relax contracted circular muscle strips. No evidence for relaxing 5-HT(4) receptors, either on inhibitory neurons or on the muscle cells was thus obtained in pig colon descendens circular muscle.

Neuronal stimulation with 5-hydroxytryptamine 4 receptor induces anti-inflammatory actions via α7nACh receptors on muscularis macrophages associated with postoperative ileus

BACKGROUND

The main symptom of postoperative ileus (POI) is an intestinal motility disorder in which monocytes/macrophages and neutrophils play crucial roles. Prokinetic 5-hydroxytryptamine 4 receptor (5-HT₄R) agonists and dopamine receptor antagonists are potential therapeutic agents for directly ameliorating the motility disorder associated with POI.

AIM

To determine the effects of the 5-HT₄R agonists mosapride citrate (MOS) and CJ-033466 on intestinal smooth muscle contractility relative to immune reactions after POI.

METHODS

Intestinal manipulation (IM) was applied to the rat distal ileum. Both MOS (0.3 and 1 mg/kg, s.c.) and CJ-033466 (1 mg/kg, s.c.) were administered to the animals before and after IM. At 24 h after IM, isolated intestinal smooth muscle contractile activity in vitro, gastrointestinal transit in vivo, inflammatory mediator expression and leucocyte infiltration were measured.

RESULTS

After IM, ileal circular muscle contractility in vitro and gastrointestinal transit in vivo were reduced and the number of macrophages and neutrophils increased in the inflamed muscle layer, resulting in the induction of inflammatory mediators such as interleukin 1 β (IL-1β), IL-6, tumour necrosis factor α (TNFα), monocyte chemoattractant protein 1 (MCP-1) and inducible nitric oxide synthase (iNOS). Both MOS and CJ-033466 significantly attenuated not only the intestinal motility dysfunction but also the leucocyte infiltration and inflammatory mediator expression after IM. The autonomic ganglionic blocker hexamethonium (1 mg/kg, i.p.) and the α7-nicotinic acetylcholine receptor (α7nAChR) antagonist methyl lycaconitine citrate (0.087 mg/kg, i.p.) blocked MOS-mediated ameliorative actions. Immunohistochemically, α7nAChR is expressed by monocytes/macrophages but not by neutrophils in the inflamed intestine.

CONCLUSION

Stimulating the 5-HT₄R accelerates acetyl choline (ACh) release from cholinergic myenteric neurons, which subsequently activates α7nAChR on activated monocytes/macrophages to inhibit their inflammatory reactions in the muscle layer. In addition to their gastroprokinetic action, 5-HT₄R agonists might serve as novel therapeutic agents for POI characterised by anti-inflammatory potency.

Constipation, IBs and the 5-HT4 Receptor: What Role for Prucalopride?

After the problems associated with the non-selective 5-HT4 receptor agonists cisapride and tegaserod, the 5-HT4 receptor is now beginning to come in from the cold. Thus, prucalopride is now the first of a new class of drug defined by selectivity and high intrinsic activity at the 5-HT4 receptor. Prucalopride has been developed for treatment of chronic constipation rather than constipation-predominant irritable bowel syndrome (IBS). This follows the trend of first evaluating new gastrointestinal (GI) prokinetic drugs in disorders where disrupted GI motility is known to exist, rather than in a functional bowel disorder where changes in motility are uncertain. If prucalopride is not progressed towards the IBS indication, it has at least shown the way for other selective 5-HT4 receptor agonists. Most notable among these is TD-5108 (velusetrag), also characterized by good selectivity at the 5-HT4 receptor, high intrinsic activity and efficacy in patients with chronic constipation.

Human 5-HT(4) and 5-HT(7) receptor splice variants: are they important?

G-protein-coupled receptors (GPCRs), which are encoded by >300 genes in the human genome, are by far the largest class of targets for modern drugs. These macromolecules display inherent adaptability of function, which is partly due to the production of different forms of the receptor protein. These are commonly called 'isoforms' or 'splice variants' denoting the molecular process of their production/assembly. Not all GPCRs are expressed as splice variants, but certain subclasses of 5-HT receptors are for example, the 5-HT(4) and 5-HT(7) receptors. There are at least 11 human 5-HT(4) and three h5-HT(7) receptor splice variants. This review describestheir discoveries, nomenclature and structures. The discovery that particular splice variants are tissue specific (or prominent) has highlighted their potential as future drug targets. In particular, this review examines the functional relevance of different 5-HT(4) and 5-HT(7) receptor splice variants. Examples are given to illustrate that splice variants have differential modulatory influences on signalling processes. Differences in agonist potency and efficacies and also differences in desensitisation rates to 5-HT occur with both 5-HT(4) and 5-HT(7) receptor splice variants. The known and candidate signalling systems that allow for splice variant specific responses include GPCR interacting proteins (GIPs) and GPCR receptor kinases (GRKs) which are examined.Finally, the relevance of 5-HT receptor splice variants to clinical medicine and to the pharmaceutical industry is discussed.

New-generation 5-HT4 receptor agonists: potential for treatment of gastrointestinal motility disorders

IMPORTANCE OF THE FIELD

Gastrointestinal (GI) dysmotility is an important mechanism in functional GI disorders (FGIDs) including constipation, irritable bowel syndrome, functional dyspepsia, and gastroparesis. 5-hydroxytryptamine(4) (5-HT(4)) receptors are targets for the treatment of GI motility disorders. However, older 5-HT(4) receptor agonists had limited clinical success because they were associated with changes in the function of the cardiac HERG potassium channel.

AREAS COVERED IN THIS REVIEW

We conducted a PubMed search using the following key words alone or in combination: 5-HT(4), safety, toxicity, pharmacokinetics, pharmacodynamics, clinical trial, cardiac, hERG, arrhythmia, potassium current, elderly, prucalopride, ATI-7505, and velusetrag (TD-5108), to review mechanisms of action, clinical efficacy, safety and tolerability of three new-generation 5-HT(4) receptor agonists.

WHAT THE READER WILL GAIN

Prucalopride, ATI-7505, and velusetrag (TD-5108) are highly selective, high-affinity 5-HT(4) receptor agonists that are devoid of action on other receptors within their therapeutic range. Their efficacy has been demonstrated in pharmacodynamic studies which demonstrate acceleration of colonic transit and, to a variable degree, in clinical trials that significantly relieve chronic constipation. Currently available evidence shows that the new 5-HT(4) receptor agonists have safe cardiac profiles.

TAKE HOME MESSAGE

New-generation 5-HT(4) receptor agonists and future drugs targeting organ-specific splice variants are promising approaches to treat GI dysmotility, particularly colonic diseases.

Role of prucalopride, a serotonin (5-HT(4)) receptor agonist, for the treatment of chronic constipation

Constipation affects up to a quarter of the population in developed countries and is associated with poor quality of life and significant economic burden. Many patients with chronic constipation are dissatisfied with current therapy due to lack of long-term efficacy or side effects. Previous nonselective 5-hydroxytryptamine receptor 4 (5-HT₄) agonists have been associated with significant interactions with other receptors (5-HT_1B, 5-HT_1D, and 5-HT_2B for tegaserod; hERG for cisapride), leading to adverse cardiovascular events resulting in withdrawal of these drugs from the market. Prucalopride is a novel gastrointestinal prokinetic agent. It acts as a high affinity, highly-selective 5-HT₄ agonist. Its efficacy in patients with chronic constipation has been demonstrated in several phase II and phase III clinical trials showing significant improvements in bowel transit, bowel function, gastrointestinal symptoms, and quality of life, with benefit maintained for up to 24 months in open label, multicenter, follow-up studies. Prucalopride’s high selectivity for the 5-HT₄ receptor may explain its favorable safety and tolerability profiles, even in elderly subjects with stable cardiovascular disease. Prucalopride is a well tolerated and efficacious prokinetic medication that should enhance the treatment of chronic constipation unresponsive to first-line treatments.

Influence of 5-HT4 receptor activation on acetylcholine release in human large intestine with endometriosis

BACKGROUND

The 5-HT(4) receptor agonist prucalopride enhances large intestinal contractility by facilitating acetylcholine release through activation of 5-HT(4) receptors on cholinergic nerves and is effective in patients with constipation. Patients with intestinal endometriosis can present with constipation. We investigated in vitro whether large intestinal endometriotic infiltration influences contractility and facilitation of acetylcholine release by prucalopride.

METHODS

Sigmoid colon or rectum circular muscle strips were obtained at the level of an endometriotic nodule with infiltration of the Auerbach plexus, and at a macroscopically healthy site at least 5 cm cranially from the nodule, in patients undergoing laparoscopic colorectal resection because of symptomatic bowel endometriosis. Responses to muscarinic receptor stimulation and to electrical field stimulation (EFS), and the facilitating effect of prucalopride on acetylcholine release were evaluated.

KEY RESULTS

The EC50 and E(max) of the contractile responses to the muscarinic receptor agonist carbachol did not differ between healthy and lesioned strips. EFS-induced on-contractions were not different between the healthy and lesioned strips, while the non-nitrergic relaxant responses induced by EFS were decreased in the lesioned strips. The facilitating effect of prucalopride on acetylcholine release in healthy strips was similar to that reported before in macroscopically healthy colon tissue of patients with colon cancer; in lesioned strips, the effect of prucalopride was fully maintained in 6/8 patients and absent in two.

CONCLUSIONS & INFERENCES

Large intestinal endometriosis does not lead to a systematic interference with the cholinergic facilitating effect of prucalopride.

Increased colonic transit in rats produced by a combination of a cholinesterase inhibitor with a 5-HT4 receptor agonist

Increased cholinergic stimulation and accelerated gastrointestinal (GI) transit may be produced by direct stimulation of the acetylcholine (ACh) receptor with an appropriate agonist by increased release of ACh from cholinergic nerve terminals or by a decreased removal or breakdown of ACh within cholinergic synapses. The acetylcholinesterase inhibitor, neostigmine, and the 5-HT(4) receptor partial agonist tegaserod, are two agents with known prokinetic activity which work by different mechanisms that result in increased levels of ACh at cholinergic synapses innervating intestinal smooth muscle. Here, we aimed to investigate the potential synergistic effect on colonic transit that may occur with concomitant use of these two agents. Colonic transit was indirectly assessed in rats via measurements of fecal pellet output every 30 min for 2.5 h following administration of neostigmine (0.003-0.1 mg kg(-1), i.p.), tegaserod (0.01-1.0 mg kg(-1), i.p.), or a combination of both compounds. When administered alone, neostigmine or tegaserod caused a dose-dependent increase in fecal pellet output. In combination, low doses of the two agents which did not produce statistically significant effects alone, compared to the vehicle, caused a significant increase in fecal pellet output. Combinations of higher doses of neostigmine and tegaserod did not display synergy. In summary, when combined at low doses, neostigmine and tegaserod produce synergistic effects manifested by a statistically significant increase in the expulsion of fecal pellets. A combination of an anticholinesterase agent with a 5-HT(4) receptor agonist may prove to be a useful therapeutic approach to treat conditions associated with slow GI transit.

Prucalopride: a new drug for the treatment of chronic constipation

Prucalopride belongs to a novel class of 5-hydroxytryptamine-4 receptor agonists, and has been evaluated extensively for the treatment of chronic constipation. Prucalopride has a stimulatory effect on gastrointestinal motility and transit, as established by in vivo and in vitro studies in animals and humans. Its therapeutic efficacy, tolerability and safety have been evaluated in Phase II and Phase III studies in chronic constipation. The cardiovascular safety profile of the drug was studied in vitro and in vivo in animal studies, in clinical studies in chronic constipation patients, as well as in specific additional clinical cardiovascular studies. Phase II studies identified a dose-dependent effect of prucalopride on bowel pattern and associated symptoms in chronic constipation. The Phase III studies mainly recruited patients with insufficient response to laxatives, and showed consistent efficacy and excellent tolerability for prucalopride.

The effects of daikenchuto (DKT) on propulsive motility in the colon

BACKGROUND

The purpose of this study is to examine the use of daikenchuto (DKT), a traditional Japanese medicine, as a potential treatment for opiate-induced slowing of intestinal transit in an isolated guinea pig colon model of motility.

METHODS

Isolated segments of distal guinea pig colon were mounted in a perfusion chamber and imaged with a digital video camera interfaced with a computer. Fecal pellets were inserted into the oral end of the colonic segment and the rates of propulsive motility over a 3 to 4 cm segment of colon were determined in the presence and absence of test compounds. In addition, intracellular recordings were obtained from intact circular muscle, and the responsiveness of inhibitory and excitatory junction potentials to DKT was evaluated.

RESULTS

The addition of D-Ala2, N-Me-Phe4, Gly-ol5 (DAMGO), a selective μ-receptor agonist, caused a concentration dependent decrease in colon motility. Naloxone did not affect basal activity, but partially restored motility in the DAMGO treated preparations. DKT (1 × 10(-4)-3 × 10(-4)g/mL) also reversed the inhibitory effect of DAMGO treated colon in a concentration dependent manner. At higher concentrations (1 × 10(-3)-3 × 10(-3)g/mL), however, this effect was lost. Motility slowed even further when naloxone and DKT were combined with noticeable disruptions in spatiotemporal patterns. Interestingly, when added alone, DKT resulted in reverse peristalsis of the pellet. In electrophysiologic studies DKT inhibited both excitatory and inhibitory junction potentials.

CONCLUSIONS

DKT appears to be as effective as naloxone in restoring motility in DAMGO treated colon. These two agents, however, do not appear to have an additive effect. When used on untreated colon segments, DKT appears to cause disruptions in the intrinsic reflex circuit of the gut resulting in a disruption of neuromuscular communication.

Therapeutic advances in functional gastrointestinal disease: irritable bowel syndrome

Reported prevalence rates of irritable bowel syndrome (IBS) are between 8% to 20% in the US general population with an average medical expenditure of US$1.35 billion direct and US$205 million indirect costs. Current pathophysiologic theories are based on abnormalities of both the brain and gut, thus setting a new stage for current and future therapeutic approaches. There are numerous treatment options in IBS acting centrally and peripherally by influencing motility and visceral sensitivity. Clinical evidence is variable; however, newer emerging treatments are being evaluated using better-designed clinical trials. Accurate assessment of IBS drug efficacy is still hampered by heterogeneity of the IBS population. Novel methods such as pharmacogenomics or brain imaging may be helpful in the future to better understand and characterize IBS patient subtypes, and this in turn will lead to more specific and efficient therapeutic options. Patient subpopulation measurement of side effects is also a clinical challenge and further understanding could improve treatment efficacy enhancing the patient compliance.