5-HT3 and 5-HT4 antagonists inhibit peristaltic contractions in guinea-pig distal colon by mechanisms independent of endogenous 5-HT

Neural targets of the enteric dopaminergic system in regulating motility of rat proximal colon

In isolated segments of the rat proximal colon, the dopamine reuptake inhibitor GBR 12909 (GBR) causes a dilatation, while the D1-like receptor antagonist SCH 23390 (SCH) induces a tonic constriction, suggesting that neurally released dopamine tonically stimulates enteric inhibitory efferent neurons. Here, the targets of the enteric dopaminergic neurons were investigated. Cannulated segments of rat proximal colon were bathed in physiological salt solution and luminally perfused with 0.9% saline, while all drugs were applied to the bath. Spatio-temporal maps of colonic motility were constructed from video recordings of peristaltic contractions, and the maximum diameter was measured as an index of colonic contractility. GBR (1 μM)-induced dilatations of colonic segments were prevented by SCH (5 μM), L-nitro arginine (L-NA; 100 μM), a nitric oxide synthase inhibitor, or tetrodotoxin (0.6 μM). In contrast, constrictions induced by a higher concentration of SCH (20 μM) were unaffected by either L-NA or tetrodotoxin. The vasoactive intestinal peptide (VIP) receptor antagonist VIP10-28 (3 μM) or P2Y1 receptor antagonist MRS 2500 (1 μM) had no effect on either the GBR-induced dilatation or the SCH-induced constriction. In colonic segments that had been pretreated with 6-hydroxydopamine (100 μM, 3 h) to deplete enteric dopamine, GBR failed to increase the colonic diameter, while SCH was still capable of constricting colonic segments. Enteric dopaminergic neurons appear to project to nitrergic neurons to dilate the proximal colon by activating neuronal D1-like receptors. In addition, constitutively activated D1-like receptors expressed in cells yet to be determined may provide a tonic inhibition on colonic constrictions.

Engineered 5-HT producing gut probiotic improves gastrointestinal motility and behavior disorder

Slow transit constipation is an intractable constipation with unknown aetiology and uncertain pathogenesis. The gut microbiota maintains a symbiotic relationship with the host and has an impact on host metabolism. Previous studies have reported that some gut microbes have the ability to produce 5-hydroxytryptamine (5-HT), an important neurotransmitter. However, there are scarce data exploiting the effects of gut microbiota-derived 5-HT in constipation-related disease. We genetically engineered the probiotic Escherichia coli Nissle 1917 (EcN-5-HT) for synthesizing 5-HT in situ. The ability of EcN-5-HT to secrete 5-HT in vitro and in vivo was confirmed. Then, we examined the effects of EcN-5-HT on intestinal motility in a loperamide-induced constipation mouse model. After two weeks of EcN-5-HT oral gavage, the constipation-related symptoms were relieved and gastrointestinal motility were enhanced. Meanwhile, administration of EcN-5-HT alleviated the constipation related depressive-like behaviors. We also observed improved microbiota composition during EcN-5-HT treatment. This work suggests that gut microbiota-derived 5-HT might promise a potential therapeutic strategy for constipation and related behavioral disorders.

Distinguishing the contributions of neuronal and mucosal serotonin in the regulation of colonic motility

BACKGROUND

Specialized enterochromaffin (EC) cells within the mucosal lining of the gut synthesize and secrete almost all serotonin (5-hydroxytryptamine, 5-HT) in the body. Significantly lower amounts of 5-HT are made by other peripheral tissues and serotonergic neurons within the enteric nervous system (ENS). EC cells are in close proximity to 5-HT receptors in the ENS, and the role of 5-HT as a modulator of gut motility, particularly colonic motor complexes, has been well defined. However, the relative contribution of neuronal 5-HT to this process under resting and stimulus-evoked conditions is unclear.

METHODS

In this study, we combined the use of the selective serotonin transporter (SERT) inhibitor, fluoxetine, with two models of mucosal 5-HT depletion-surgical removal of the mucosa and our Tph1^Cre/ERT2 ; Rosa26^DTA mouse line-to determine the relative contribution of neuronal and mucosal 5-HT to resting and distension-evoked colonic motility.

KEY RESULTS

Fluoxetine significantly reduced the frequency of colonic migrating complexes (CMCs) in flat-sheet preparations with the mucosa present and in intact control Tph1-DTA colons in which EC cells were present. No such effect was observed in mucosa-free preparations or in intact Tph1-DTA preparations lacking EC cell 5-HT.

CONCLUSIONS AND INFERENCES

We demonstrate that mucosal 5-HT release plays an important role in distension-evoked colonic motility, and that SERT inhibition no longer alters gut motility when EC cells are absent, thus demonstrating that ENS 5-HT does not play a role in regulating gut motility.

Role of 5-HT in the enteric nervous system and enteroendocrine cells

Since the 1950s, considerable circumstantial evidence had been presented that endogenous 5-HT (serotonin) synthesized from within the wall of the gastrointestinal (GI) tract played an important role in GI motility and transit. However, identifying the precise functional role of gut-derived 5-HT has been difficult to ascertain, for a number of reasons. Over the past decade, as recording techniques have advanced significantly and access to new genetically modified animals improved, there have been major new insights and major changes in our understanding of the functional role of endogenous 5-HT in the GI tract. Data from many different laboratories have shown that major patterns of GI motility and transit still occur with minor or no, change when all endogenous 5-HT is pharmacologically or genetically ablated from the gut. Furthermore, antagonists of 5-HT3 receptors are equally, or more potent at inhibiting GI motility in segments of intestine that are completely depleted of endogenous 5-HT. Here, the most recent findings are discussed with regard to the functional role of endogenous 5-HT in enterochromaffin cells and enteric neurons in gut motility and more broadly in some major homeostatic pathways.

The Shaggy Dog Story of Enteric Signaling: Serotonin, a Molecular Megillah

Historically and quantitatively, the enteric site of serotonin (5-HT) storage has primacy over those of any other organ. 5-HT, by the name of "enteramine", was first discovered in the bowel, and the gut produces most of the body's 5-HT. Not only does the bowel secrete 5-HT prodigiously but it also expresses a kaleidoscopic abundance of 5-HT receptors. The larger of two enteric 5-HT stores is mucosal, biosynthetically dependent upon tryptophan hydroxylase1 (TPH1), and located in EC cells. Mechanical stimuli, nutrients, luminal bacteria, and neurotransmitters such as acetylcholine and norepinephrine are all able to stimulate EC cells. Paracrine actions of 5-HT allow the mucosa to signal to neurons to initiate peristaltic and secretory reflexes as well as to inflammatory cells to promote intestinal inflammation. Endocrine effects of 5-HT allow EC cells to influence distant organs, including bone, liver, and endocrine pancreas. The smaller enteric 5-HT store is biosynthetically dependent upon TPH2 and is located within a small subset of myenteric neurons. 5-HT is responsible for slow excitatory neurotransmission manifested primarily in type II/AH neurons. Importantly, neuronal 5-HT also promotes enteric nervous system (ENS) neurogenesis, both pre- and postnatally, through 5-HT_2B and especially 5-HT₄ receptors. The early birth of serotonergic neurons allows these cells to function as sculptors of the mature ENS. The inactivation of secreted 5-HT depends on transmembrane transport mediated by a serotonin transporter (SERT; SLC6A4). The importance of SERT in control of 5-HT's function means that pharmacological inhibition of SERT, as well as gain- or loss-of-function mutations in SLC6A4, can exert profound effects on development and function of the ENS. Extra-enteric, TPH1-derived 5-HT from yolk sac and placenta promotes neurogenesis before enteric neurons synthesize 5-HT and contribute to ENS patterning. The impressive multi-functional nature of enteric 5-HT has made the precise identification of individual physiological roles difficult and sometimes controversial.

The α isoform of cGMP-dependent protein kinase 1 (PKG1α) is expressed and functionally important in intrinsic primary afferent neurons of the guinea pig enteric nervous system

BACKGROUND

Intrinsic primary afferent neurons (IPANs) enable the gut to manifest reflexes in the absence of CNS input. PKG1α is selectively expressed in a subset of neurons in dorsal root ganglia (DRG) and has been linked to nociception and long-term hyperexcitability.

METHODS

We used immunoblotting, immunocytochemistry, and in vitro assays of IPAN-dependent enteric functions to test hypotheses that subsets of primary neurons of the ENS and DRG share a reliance on PKG1α expression.

KEY RESULTS

PKG1α immunoreactivity was demonstrated in immunoblots from isolated myenteric ganglia. PKG1α, but not PKG1β, immunoreactivity, was coincident with that of neuronal markers (HuC/D; β3-tubulin) in both enteric plexuses. PKG1α immunoreactivity also co-localized with the immunoreactivities of the IPAN markers, calbindin (100%; myenteric plexus) and cytoplasmic NeuN (98 ± 1% submucosal plexus). CGRP-immunoreactive DRG neurons, identified as visceral afferents by retrograde transport, were PKG1α-immunoreactive. We used intraluminal cholera toxin to determine whether PKG1α was necessary to enable stimulation of the mucosa to activate Fos in enteric neurons. Tetrodotoxin (1.0 µM), low Ca²⁺ /high Mg²⁺ media, and the PKG inhibitor, N46 (100 µM), all inhibited Fos activation in myenteric neurons. N46 also concentration dependently inhibited peristaltic reflexes in isolated preparations of distal colon (IC₅₀  = 83.3 ± 1.3 µM).

CONCLUSIONS & INFERENCES

These data suggest that PKG1α is present and functionally important in IPANs and visceral afferent nociceptive neurons.

Microbial signalling in colonic motility

Sensory nerve endings within the wall of the gastrointestinal (GI) tract may respond to bacterial signalling, providing the basis for key biological processes that underlie intestinal motility and microbial homeostasis. Enteric neurons and smooth muscle cells are well known to express an array of receptors, including G-protein coupled receptors and ligand-gated ion channels, that can sense chemical ligands and other bacterially-derived substances. These include short chain fatty acids, secondary bile acids and lipopolysaccharide. For neural detection of microbial activators to occur, luminal substances must first interact with enterocytes for direct signalling or cross paracellularly. Recent studies indicate that bacterial-derived microvesicles can cross the gut epithelial barrier and affect motility. This suggests a possible intercellular communication pathway between the GI tract and the ENS. We explore the idea that bacterial microvesicles can behave as a delivery package for communication between microbe and host.

Does Intravenous Ondansetron Affect the Intestinal Motility Pattern in Healthy Donkeys (Equus asinus)?

The current study was undertaken to investigate the effect of intravenous administration of ondansetron on the small and large intestinal motility in donkeys (Equus asinus) using non-invasive transabdominal ultrasonography. The current prospective, randomized, placebo-controlled, crossover study was conducted on thirty healthy donkeys (15 males and 15 females). The selected donkeys underwent two trials; the first was performed by intravenous administration of saline solution as a placebo, while the second was carried out by intravenous administration of ondansetron hydrochloride. The contractility of selected portions of both the small intestine (duodenum and jejunum) and the large intestine (left colon, right colon, and cecum) was counted over a period of 3 minutes before administration (zero time) and at 15, 30, 60, 90, 120, 180, and 240 minutes after administration. The results of this study showed that ondansetron significantly altered the small and large intestinal contractility compared to normal saline. Intravenous administration of ondansetron induced a significant decrease in the duodenal, jejunal and cecal contractility compared to placebo at 30, 60, 90, and 120 minutes after administration. Likewise, ondansetron induced a significant decrease in the left colon and right colon contractility when compared with placebo at 30, 60, 90, 120, and 180 minutes following administration. Ondansetron can be used as a highly specific and selective serotonin 5-HT₃ receptor antagonist for reducing the small and large intestinal motility in donkeys, and is therefore highly suggested for treating spasmodic colic in equine.

Gut bacteria-derived 5-hydroxyindole is a potent stimulant of intestinal motility via its action on L-type calcium channels

Microbial conversion of dietary or drug substrates into small bioactive molecules represents a regulatory mechanism by which the gut microbiota alters intestinal physiology. Here, we show that a wide variety of gut bacteria can metabolize the dietary supplement and antidepressant 5-hydroxytryptophan (5-HTP) to 5-hydroxyindole (5-HI) via the tryptophanase (TnaA) enzyme. Oral administration of 5-HTP results in detection of 5-HI in fecal samples of healthy volunteers with interindividual variation. The production of 5-HI is inhibited upon pH reduction in in vitro studies. When administered orally in rats, 5-HI significantly accelerates the total gut transit time (TGTT). Deciphering the underlying mechanisms of action reveals that 5-HI accelerates gut contractility via activation of L-type calcium channels located on the colonic smooth muscle cells. Moreover, 5-HI stimulation of a cell line model of intestinal enterochromaffin cells results in significant increase in serotonin production. Together, our findings support a role for bacterial metabolism in altering gut motility and lay the foundation for microbiota-targeted interventions.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Novel aspects of enteric serotonergic signaling in health and brain-gut disease

Gastrointestinal (GI) comorbidities are common in individuals with mood and behavioral dysfunction. Similarly, patients with GI problems more commonly suffer from co-morbid psychiatric diagnoses. Although the central and enteric nervous systems (CNS and ENS, respectively) have largely been studied separately, there is emerging interest in factors that may contribute to disease states involving both systems. There is strong evidence to suggest that serotonin may be an important contributor to these brain-gut conditions. Serotonin has long been recognized for its critical functions in CNS development and function. The majority of the body's serotonin, however, is produced in the GI tract, where it plays key roles in ENS development and function. Further understanding of the specific impact that enteric serotonin has on brain-gut disease may lay the foundation for the creation of novel therapeutic targets. This review summarizes the current data focusing on the important roles that serotonin plays in ENS development and motility, with a focus on novel aspects of serotonergic signaling in medical conditions in which CNS and ENS co-morbidities are common, including autism spectrum disorders and depression.

Muscarinic suppression of ATP-sensitive K+ channels mediated by the M3/Gq/11/phospholipase C pathway contributes to mouse ileal smooth muscle contractions

ATP-sensitive K⁺ (K_ATP) channels are expressed in gastrointestinal smooth muscles, and their activity is regulated by muscarinic receptor stimulation. However, the physiological significance and mechanisms of muscarinic regulation of K_ATP channels are not fully understood. We examined the effects of the K_ATP channel opener cromakalim and the K_ATP channel blocker glibenclamide on electrical activity of single mouse ileal myocytes and on mechanical activity in ileal segment preparations. To explore muscarinic regulation of K_ATP channel activity and its underlying mechanisms, the effect of carbachol (CCh) on cromakalim-induced K_ATP channel currents ( I_KATP) was studied in myocytes of M₂ or M₃ muscarinic receptor-knockout (KO) and wild-type (WT) mice. Cromakalim (10 µM) induced membrane hyperpolarization in single myocytes and relaxation in segment preparations from WT mice, whereas glibenclamide (10 µM) caused membrane depolarization and contraction. CCh (100 µM) induced sustained suppression of I_KATP in cells from both WT and M₂KO mice. However, CCh had a minimal effect on I_KATP in M₃KO and M₂/M₃ double-KO cells. The G_q/11 inhibitor YM-254890 (10 μM) and PLC inhibitor U73122 (1 μM), but not the PKC inhibitor calphostin C (1 μM), markedly decreased CCh-induced suppression of I_KATP in WT cells. These results indicated that K_ATP channels are constitutively active and contribute to the setting of resting membrane potential in mouse ileal smooth muscles. M₃ receptors inhibit the activity of these channels via a G_q/11/PLC-dependent but PKC-independent pathways, thereby contributing to membrane depolarization and contraction of smooth muscles. NEW & NOTEWORTHY We systematically investigated the regulation of ATP-sensitive K⁺ channels by muscarinic receptors expressed on mouse ileal smooth muscles. We found that M₃ receptors inhibit the activity of ATP-sensitive K⁺ channels via a G_q/11/PLC-dependent, but PKC-independent, pathway. This muscarinic suppression of ATP-sensitive K⁺ channels contributes to membrane depolarization and contraction of smooth muscles.

Recent advances in pharmacological research on the management of irritable bowel syndrome

Irritable bowel syndrome (IBS), a common gastrointestinal (GI) disorder, is associated with various factors, including lifestyle, infection, stress, intestinal flora, and related diseases. The pharmacotherapeutic stimulation of receptors and downstream signaling pathways is effective in reducing IBS symptoms; however, it is still associated with adverse effects. Various receptors related to GI motility and visceral hypersensitivity should be considered to enhance the benefit/risk ratio of IBS treatments. This review discusses recent pharmacological advances in IBS management. Several receptors related to GI motility and abdominal pain are investigated in various angles. 5-Hydroxytryptamine (5-HT) is an important neurotransmitter that activates the colonic mucosal 5-HT₄ receptor without causing severe cardiovascular adverse effects. The clinical potential of ramosetron for diarrhea-predominant IBS has been suggested because of a lower risk of ischemic colitis than conventional 5-HT₃ receptor antagonists. Toll-like receptors (TLRs), especially TLR2 and TLR4, show a significant effect on the post-infection symptoms and lipopolysaccharide-mediated regulation of GI motility. Histamine is a well-known nitrogenous compound that regulates inflammatory responses and visceral hypersensitivity. Histamine 1 receptor-mediated sensitization of the transient receptor potential vanilloid 1 is associated with IBS. Pharmacological approaches based on these signaling pathways could be useful in the development of novel IBS treatments.

Exogenous serotonin regulates colorectal motility via the 5-HT2 and 5-HT3 receptors in the spinal cord of rats

BACKGROUND

We previously reported that intrathecal injection of noradrenaline or dopamine causes enhancement of colorectal motility. As these monoamines are neurotransmitters of descending pain inhibitory pathways in the spinal cord, we hypothesized that serotonin, which is one of the neurotransmitters involved in descending pain inhibition, also influences the lumbosacral defecation center. Therefore, we examined whether serotonin acting on the spinal defecation center enhances colorectal motility.

METHODS

Colorectal intraluminal pressure and propelled liquid volume were recorded in vivo in anesthetized rats.

KEY RESULTS

Intrathecal injection of serotonin into the L6-S1 spinal cord elicited periodic increases in colorectal intraluminal pressure, being associated with increases in liquid output. Pharmacological experiments revealed that the effect of serotonin is mediated by both 5-HT₂ and 5-HT₃ receptors. The serotonin-induced enhancement of colorectal motility was unaffected even after disconnection of the defecation center from supraspinal regions by cutting the T8 spinal cord, while transection of the parasympathetic pelvic nerves prevented the colokinetic effect of serotonin. Finally, we investigated interactions among serotonin, noradrenaline and dopamine. Simultaneous administration of sub-effective doses of these monoamine neurotransmitters into the spinal cord caused propulsive colorectal motility slightly but substantially.

CONCLUSIONS AND INFERENCES

These results demonstrate that exogenous serotonin acts on 5-HT₂ and 5-HT₃ receptors in the lumbosacral defecation center and activates the parasympathetic nervous system to enhance colorectal motility in cooperation with noradrenaline and dopamine.

A model of the enteric neural circuitry underlying the generation of rhythmic motor patterns in the colon: the role of serotonin

We discuss the role of multiple cell types involved in rhythmic motor patterns in the large intestine that include tonic inhibition of the muscle layers interrupted by rhythmic colonic migrating motor complexes (CMMCs) and secretomotor activity. We propose a model that assumes these motor patterns are dependent on myenteric descending 5-hydroxytryptamine (5-HT, serotonin) interneurons. Asynchronous firing in 5-HT neurons excite inhibitory motor neurons (IMNs) to generate tonic inhibition occurring between CMMCs. IMNs release mainly nitric oxide (NO) to inhibit the muscle, intrinsic primary afferent neurons (IPANs), glial cells, and pacemaker myenteric pacemaker interstitial cells of Cajal (ICC-MY). Mucosal release of 5-HT from enterochromaffin (EC) cells excites the mucosal endings of IPANs that synapse with 5-HT descending interneurons and perhaps ascending interneurons, thereby coupling EC cell 5-HT to myenteric 5-HT neurons, synchronizing their activity. Synchronized 5-HT neurons generate a slow excitatory postsynaptic potential in IPANs via 5-HT7 receptors and excite glial cells and ascending excitatory nerve pathways that are normally inhibited by NO. Excited glial cells release prostaglandins to inhibit IMNs (disinhibition) to allow full excitation of ICC-MY and muscle by excitatory motor neurons (EMNs). EMNs release ACh and tachykinins to excite pacemaker ICC-MY and muscle, leading to the simultaneous contraction of both the longitudinal and circular muscle layers. Myenteric 5-HT neurons also project to the submucous plexus to couple motility with secretion, especially during a CMMC. Glial cells are necessary for switching between different colonic motor behaviors. This model emphasizes the importance of myenteric 5-HT neurons and the likely consequence of their coupling and uncoupling to mucosal 5-HT by IPANs during colonic motor behaviors.

A sexually dimorphic effect of cholera toxin: rapid changes in colonic motility mediated via a 5-HT3 receptor-dependent pathway in female C57Bl/6 mice

KEY POINTS

Cholera causes more than 100,000 deaths each year as a result of severe diarrhoea, vomiting and dehydration due to the actions of cholera toxin; more females than males are affected. Cholera toxin induces hypersecretion via release of mucosal serotonin and over-activation of enteric neurons, but its effects on gastrointestinal motility are not well characterized. We found that cholera toxin rapidly and reversibly reduces colonic motility in female mice in oestrus, but not in males or females in prooestrus, an effect mediated by 5-HT in the colonic mucosa and by 5-HT3 receptors. We show that the number of mucosal enterochromaffin cells containing 5-HT changes with the oestrous cycle in mice. These findings indicate that cholera toxin's effects on motility are rapid and depend on the oestrous cycle and therefore can help us better understand differences in responses in males and female patients.

ABSTRACT

Extensive studies of the mechanisms responsible for the hypersecretion produced by cholera toxin (CT) have shown that this toxin produces a massive over-activation of enteric neural secretomotor circuits. The effects of CT on gastrointestinal motility, however, have not been adequately characterized. We investigated effects of luminal CT on neurally mediated motor activity in ex vivo male and female mouse full length colon preparations. We used video recording and spatiotemporal maps of contractile activity to quantify colonic migrating motor complexes (CMMCs) and resting colonic diameter. We compared effects of CT in female colon from wild-type and mice lacking tryptophan hydroxylase (TPH1KO). We also compared CMMCs in colons of female mice in oestrus with those in prooestrus. In female (but not male) colon, CT rapidly, reversibly and concentration-dependently inhibits CMMC frequency and induces a tonic constriction. These effects were blocked by granisetron (5-HT3 antagonist) and were absent from TPH1KO females. CT effects were prominent at oestrus but absent at prooestrus. The number of EC cells containing immunohistochemically demonstrable serotonin (5-HT) was 30% greater in female mice during oestrus than during prooestrus or in males. We conclude that CT inhibits CMMCs via release of mucosal 5-HT, which activates an inhibitory pathway involving 5-HT3 receptors. This effect is sex- and oestrous cycle-dependent and is probably due to an oestrous cycle-dependent change in the number of 5-HT-containing EC cells in the colonic mucosa.

The effect of Flos Lonicerae Japonicae extract on gastro-intestinal motility function

ETHNOPHARMACOLOGICAL RELEVANCE

Flos Lonicerae Japonicae is a well-known herb of traditional Chinese medicine that has been used for heat-clearing, detoxification, anti-inflammation, throat pain and gastro-intestinal (GI) disorder. In order to verify the effect of Flos Lonicerae Japonicae on GI disorder, we investigated the prokinetic effect of GC-7101 on GI motility function.

MATERIALS AND METHODS

GC-7101 is the standardized extract of Flos Lonicerae Japonicae. The contractile action of GC-7101 on feline esophageal smooth muscle cell (ESMC) was evaluated by measuring dispersed cell length. The isometric tension study was performed to investigate the effect of GC-7101 on feline lower esophageal sphincther (LES). The prokinetic effect of GC-7101 was investigated by gastric emptying (GE) and gastro-intestinal transit (GIT) in rats.

RESULTS

GC-7101 produced concentration-dependent contractions in ESMCs. Pretreatment with 5-HT3 and 5-HT4 receptor blocker (ondansetron and GR113808) inhibited the contractile responses of the GC-7101-induced ESMCs. In isometric tension study, GC-7101 recovered the HCl-induced decreased tone of LES muscle strips. The treatment of GC-7101 enhanced the carbachol-induced contractile responses and the electric field stimulation (EFS)-induced on-contraction. The oral administration of GC-7101 not only significantly accelerated GE and GIT in normal rats but also recovered the delayed GE and GIT, and its effect was more potent than that of conventional prokinetics (e.g., domperidone, a dopamine-receptor antagonist, and mosapride, a 5-HT4-receptor agonist).

CONCLUSION

GC-7101 revealed a prokinetic effect through enhancing the contractile responses of ESMCs, tone increases, enhancing the carbarchol- or EFS-induced contractile responses of LES muscle strips, and the acceleration of GE and GIT. We have identified the significant potential of GC-7101 for the development of new prokinetic drugs through this study.

Constitutively Active 5-HT Receptors: An Explanation of How 5-HT Antagonists Inhibit Gut Motility in Species Where 5-HT is Not an Enteric Neurotransmitter?

Antagonists of 5-Hydroxytryptamine (5-HT) receptors are well known to inhibit gastrointestinal (GI)-motility and transit in a variety of mammals, including humans. Originally, these observations had been interpreted by many investigators (including us) as evidence that endogenous 5-HT plays a major role in GI motility. This seemed a logical assumption. However, the story changed dramatically after recent studies revealed that 5-HT antagonists still blocked major GI motility patterns (peristalsis and colonic migrating motor complexes) in segments of intestine depleted of all 5-HT. Then, these results were further supported by Dr. Gershons' laboratory, which showed that genetic deletion of all genes that synthesizes 5-HT had minor, or no inhibitory effects on GI transit in vivo. If 5-HT was essential for GI motility patterns and transit, then one would expect major disruptions in motility and transit when 5-HT synthesis was genetically ablated. This does not occur. The inhibitory effects of 5-HT antagonists on GI motility clearly occur independently of any 5-HT in the gut. Evidence now suggests that 5-HT antagonists act on 5-HT receptors in the gut which are constitutively active, and don't require 5-HT for their activation. This would explain a long-standing mystery of how 5-HT antagonists inhibit gut motility in species like mice, rats, and humans where 5-HT is not an enteric neurotransmitter. Studies are now increasingly demonstrating that the presence of a neurochemical in enteric neurons does not mean they function as neurotransmitters. Caution should be exercised when interpreting any inhibitory effects of 5-HT antagonists on GI motility.

Colonic migrating motor complexes, high amplitude propagating contractions, neural reflexes and the importance of neuronal and mucosal serotonin

The colonic migrating motor complex (CMMC) is a critical neurally mediated rhythmic propulsive contraction observed in the large intestine of many mammals. It seems to be equivalent to the high amplitude propagating contractions (HAPCs) in humans. This review focuses on the probable neural mechanisms involved in producing the CMMC or HAPC, their likely dependence on mucosal and neuronal serotonin and pacemaker insterstitial cells of Cajal networks and how intrinsic neural reflexes affect them. Discussed is the possibility that myenteric 5-hydroxytryptamine (5-HT) neurons are not only involved in tonic inhibition of the colon, but are also involved in generating the CMMC and modulation of the entire enteric nervous system, including coupling motility to secretion and blood flow. Mucosal 5-HT appears to be important for the initiation and effective propagation of CMMCs, although this mechanism is a longstanding controversy since the 1950s, which we will address. We argue that the slow apparent propagation of the CMMC/HAPC down the colon is unlikely to result from a slowly conducting wave front of neural activity, but more likely because of an interaction between ascending excitatory and descending (serotonergic) inhibitory neural pathways interacting both within the myenteric plexus and at the level of the muscle. That is, CMMC/HAPC propagation appears to be similar to esophageal peristalsis. The suppression of inhibitory (neuronal nitric oxide synthase) motor neurons and mucosal 5-HT release by an upregulation of prostaglandins has important implications in a number of gastrointestinal disorders, especially slow transit constipation.

Extensive projections of myenteric serotonergic neurons suggest they comprise the central processing unit in the colon

BACKGROUND

5-Hydroxytryptamine (5-HT, serotonin) is an important regulator of colonic motility and secretion; yet the role of serotonergic neurons in the colon is controversial.

METHODS

We used immunohistochemical techniques to examine their projections throughout the enteric nervous system and interstitial cells of Cajal (ICC) networks in the murine proximal to mid colon.

KEY RESULTS

Serotonergic neurons, which were mainly calbindin positive, occurred only in myenteric ganglia (1 per 3 ganglia). They were larger than nNOS neurons but similar in size to Dogiel Type II (AH) neurons. 5-HT neurons, appeared to make numerous varicose contacts with each other, most nNOS neurons, Dogiel Type II/AH neurons and glial cells. 5-HT, calbindin and nNOS nerve fibers also formed a thin perimuscular nerve plexus that was associated with ganglia, which contained both nNOS positive and negative neurons, which lay directly upon the submucosal pacemaker ICC network. Neurons in perimuscular ganglia were surrounded by 5-HT varicosities. Submucous ganglia contained nNOS positive and negative neurons, and calbindin positive neurons, which also appeared richly supplied by serotonergic nerve varicosities. Serotonergic nerve fibers ran along submucosal arterioles, but not veins. Varicosities of serotonergic nerve fibers were closely associated with pacemaker ICC networks and with intramuscular ICC (ICC-IM). 5-HT2B receptors were found on a subpopulation of non-5-HT containing myenteric neurons and their varicosities, pacemaker ICC-MY and ICC-IM.

CONCLUSIONS & INFERENCES

Myenteric serotonergic neurons, whose axons exhibit considerable divergence, regulate the entire enteric nervous system and are important in coordinating motility with secretion. They are not just interneurons, as regularly assumed, but possibly also motor neurons to ICC and blood vessels, and some may even be sensory neurons.

Important role of mucosal serotonin in colonic propulsion and peristaltic reflexes: in vitro analyses in mice lacking tryptophan hydroxylase 1

Although there is general agreement that mucosal 5-hydroxytryptamine (5-HT) can initiate peristaltic reflexes in the colon, recent studies have differed as to whether or not the role of mucosal 5-HT is critical. We therefore tested the hypothesis that the secretion of 5-HT from mucosal enterochromaffin (EC) cells is essential for the manifestation of murine colonic peristaltic reflexes. To do so, we analysed the mechanisms underlying faecal pellet propulsion in isolated colons of mice lacking tryptophan hydroxylase 1 (Tph1(-/-) mice), which is the rate-limiting enzyme in the biosynthesis of mucosal but not neuronal 5-HT. We used video analysis of faecal pellet propulsion, tension transducers to record colonic migrating motor complexes (CMMCs) and intracellular microelectrodes to record circular muscle activity occurring spontaneously or following intraluminal distension. When compared with control (Tph1(+/+)) mice, Tph1(-/-) animals exhibited: (1) an elongated colon; (2) larger faecal pellets; (3) orthograde propulsion followed by retropulsion (not observed in Tph1(+/+) colon); (4) slower in vitro propulsion of larger faecal pellets (28% of Tph1(+/+)); (5) CMMCs that infrequently propagated in an oral to anal direction because of impaired descending inhibition; (6) reduced CMMCs and inhibitory responses to intraluminal balloon distension; (7) an absence of reflex activity in response to mucosal stimulation. In addition, (8) thin pellets that propagated along the control colon failed to do so in Tph1(-/-) colon; and (9) the 5-HT3 receptor antagonist ondansetron, which reduced CMMCs and blocked their propagation in Tph1(+/+) mice, failed to alter CMMCs in Tph1(-/-) animals. Our observations suggest that mucosal 5-HT is essential for reflexes driven by mucosal stimulation and is also important for normal propagation of CMMCs and propulsion of pellets in the isolated colon.