Slow excitatory post-synaptic potentials in myenteric AH neurons of the guinea-pig ileum are reduced by the 5-hydroxytryptamine7 receptor antagonist SB 269970

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Metabotropic 5-HT receptor-mediated effects in the human submucous plexus

BACKGROUND

Serotonin (5-HT) is an important mediator in the gastrointestinal tract, acting on different neuronal 5-HT receptors. The ionotropic 5-HT₃ receptor mediates immediate but transient spike discharge in human enteric neurons. We studied the role of the metabotropic 5-HT_1P , 5-HT₄ , and 5-HT₇ receptors to activate human submucous neurons.

METHODS

Neuroimaging using the voltage sensitive dye Di-8-ANEPPS was performed in submucous plexus preparations from human surgical specimens of the small and large intestine. We synthesized a new, stable 5-HT_1P agonist, 5-benzyloxyhydrazonoindalpine (5-BOHIP).

KEY RESULTS

5-HT evoked a fast and late-onset spike discharge in enteric neurons. The fast component was blocked by the 5-HT₃ receptor antagonist cilansetron, while the remaining sustained response was significantly reduced by the 5-HT_1P receptor antagonist 5-hydroxytryptophanyl-5-hydroxytryptophan amide (5-HTP-DP). The newly synthesized 5-HT_1P agonist 5-BOHIP induced a slowly developing, long-lasting activation of submucous neurons, which was blocked by 5-HTP-DP. We could not demonstrate any 5-HT₇ receptor-induced spike discharge based on the lack of response to 5-carboxamidotryptamine. Similarly, the 5-HT₄ agonists 5-methoxytryptamine and prucalopride evoked no immediate or late-onset spike discharge.

CONCLUSIONS & INFERENCES

Our work demonstrated for the first time the presence of functional 5-HT_1P receptors on human submucous neurons. Furthermore, we found no evidence for a role of 5-HT₄ or 5-HT₇ receptors in the postsynaptic activation of human submucous neurons by 5-HT.

5-HT7 receptor-dependent intestinal neurite outgrowth contributes to visceral hypersensitivity in irritable bowel syndrome

Irritable bowel syndrome (IBS) is characterized by visceral hypersensitivity (VH) associated with abnormal serotonin/5-hydroxytryptamine (5-HT) metabolism and neurotrophin-dependent mucosal neurite outgrowth. The underlying mechanisms of VH remain poorly understood. We investigated the role of 5-HT₇ receptor in mucosal innervation and intestinal hyperalgesia. A high density of mucosal nerve fibres stained for 5-HT₇ was observed in colonoscopic biopsy specimens from IBS patients compared with those from healthy controls. Staining of 5-HT₃ and 5-HT₄ receptors was observed mainly in colonic epithelia with comparable levels between IBS and controls. Visceromotor responses to colorectal distension were evaluated in two mouse models, one postinfectious with Giardia and subjected to water avoidance stress (GW) and the other postinflammatory with trinitrobenzene sulfonic acid-induced colitis (PT). Increased VH was associated with higher mucosal density of 5-HT₇-expressing nerve fibres and elevated neurotrophin and neurotrophin receptor levels in the GW and PT mice. The increased VH was inhibited by intraperitoneal injection of SB-269970 (a selective 5-HT₇ antagonist). Peroral multiple doses of CYY1005 (a novel 5-HT₇ ligand) decreased VH and reduced mucosal density of 5-HT₇-expressing nerve fibres in mouse colon. Human neuroblastoma SH-SY5Y cells incubated with bacteria-free mouse colonic supernatant, 5-HT, nerve growth factor, or brain-derived neurotrophic factor exhibited nerve fibre elongation, which was inhibited by 5-HT₇ antagonists. Gene silencing of HTR7 also reduced the nerve fibre length. Activation of 5-HT₇ upregulated NGF and BDNF gene expression, while stimulation with neurotrophins increased the levels of tryptophan hydroxylase 2 and 5-HT₇ in neurons. A positive-feedback loop was observed between serotonin and neurotrophin pathways via 5-HT₇ activation to aggravate fibre elongation, whereby 5-HT₃ and 5-HT₄ had no roles. In conclusion, 5-HT₇-dependent mucosal neurite outgrowth contributed to VH. A novel 5-HT₇ antagonist could be used as peroral analgesics for IBS-related pain.

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 Role of Serotonin Neurotransmission in Gastrointestinal Tract and Pharmacotherapy

5-Hydroxytryptamine (5-HT, serotonin) is a neurotransmitter in both the central nervous system and peripheral structures, acting also as a hormone in platelets. Although its concentration in the gut covers >90% of all organism resources, serotonin is mainly known as a neurotransmitter that takes part in the pathology of mental diseases. Serotonin modulates not only CNS neurons, but also pain transmission and platelet aggregation. In the periphery, 5-HT influences muscle motility in the gut, bronchi, uterus, and vessels directly and through neurons. Serotonin synthesis starts from hydroxylation of orally delivered tryptophan, followed by decarboxylation. Serotonin acts via numerous types of receptors and clinically plays a role in several neural, mental, and other chronic disorders, such as migraine, carcinoid syndrome, and some dysfunctions of the alimentary system. 5-HT acts as a paracrine hormone and growth factor. 5-HT receptors in both the brain and gut are targets for drugs modifying serotonin neurotransmission. The aim of the present article is to review the 5-HT receptors in the gastrointestinal (GI) tract to determine the role of serotonin in GI physiology and pathology, including known GI diseases and the role of serotonin in GI pharmacotherapy.

Colonic 5-HT4 receptors are targets for novel prokinetic drugs

5-HT₄ receptors are G protein-coupled receptors that link to the stimulatory protein Gs which activates adenylate cyclase to increase intracellular cyclic AMP which then activates protein kinase A (PKA). 5-HT₄ receptors are expressed by neurons in the central and peripheral nervous systems especially the enteric nervous system (ENS). In general, 5-HT₄ receptors are stimulatory and their activation in the ENS enhances neurotransmitter release and propulsive motility patterns. 5-HT₄ receptors are expressed by enterochromaffin (EC) cells, Goblet cells, and most enteric neurons. The study by Konen and colleagues in this issue of Neurogastroenterology and Motility features two novel 5-HT₄ receptor agonists (5-HT₄ -LA1 and 5-HT₄ -LA-2) that are not absorbed from the gastrointestinal tract of mice and act locally in the colonic mucosa to stimulate propulsive motility. The authors show that 5-HT₄ -LA1 and 5-HT₄ -LA2 were not absorbed from the colon and that both drugs stimulated colonic transit when administered by gavage. Both agonists stimulated colonic glass bead expulsion, and 5-HT₄ LA1 activation stimulated fecal output and increased fecal water content. These effects were detected in young and aged mice. 5-HT₄ receptors were also localized to the epithelium of the human duodenum, ileum, and colon. These studies highlight novel 5-HT₄ receptor agonists that have prokinetic actions on the GI tract. These drugs are not absorbed and act locally in the gut mucosa to stimulate propulsive motility while minimizing access to systemic 5-HT₄ receptors and avoiding potential unwanted side effects.

Dietary Lipid Modulation of Intestinal Serotonin in Ballan Wrasse (Labrus bergylta)-In Vitro Analyses

Serotonin (5-HT) is pivotal in the complex regulation of gut motility and consequent digestion of nutrients via multiple receptors. We investigated the serotonergic system in an agastric fish species, the ballan wrasse (Labrus bergylta) as it represents a unique model for intestinal function. Here we present evidence of the presence of enterochromaffin cells (EC cells) in the gut of ballan wrasse comprising transcriptomic data on EC markers like adra2a, trpa1, adgrg4, lmxa1, spack1, serpina10, as well as the localization of 5-HT and mRNA of the rate limiting enzyme; tryptophan hydroxylase (tph1) in the gut epithelium. Second, we examined the effects of dietary marine lipids on the enteric serotonergic system in this stomach-less teleost by administrating a hydrolyzed lipid bolus in ex vivo guts in an organ bath system. Modulation of the mRNA expression from the tryptophan hydroxylase tph1 (EC cells isoform), tph2 (neural isoform), and other genes involved in the serotonergic machinery were tracked. Our results showed no evidence to confirm that the dietary lipid meal did boost the production of 5-HT within the EC cells as mRNA tph1 was weakly regulated postprandially. However, dietary lipid seemed to upregulate the post-prandial expression of tph2 found in the serotonergic neurons. 5-HT in the intestinal tissue increased 3 hours after "exposure" of lipids, as was observed in the mRNA expression of tph2. This suggest that serotonergic neurons and not EC cells are responsible for the substantial increment of 5-HT after a lipid-reach "meal" in ballan wrasse. Cells expressing tph1 were identified in the gut epithelium, characteristic for EC cells. However, Tph1 positive cells were also present in the lamina propria. Characterization of these cells together with their implications in the serotonergic system will contribute to broad the scarce knowledge of the serotonergic system across teleosts.

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.

Enteric nervous system: sensory transduction, neural circuits and gastrointestinal motility

The gastrointestinal tract is the only internal organ to have evolved with its own independent nervous system, known as the enteric nervous system (ENS). This Review provides an update on advances that have been made in our understanding of how neurons within the ENS coordinate sensory and motor functions. Understanding this function is critical for determining how deficits in neurogenic motor patterns arise. Knowledge of how distension or chemical stimulation of the bowel evokes sensory responses in the ENS and central nervous system have progressed, including critical elements that underlie the mechanotransduction of distension-evoked colonic peristalsis. Contrary to original thought, evidence suggests that mucosal serotonin is not required for peristalsis or colonic migrating motor complexes, although it can modulate their characteristics. Chemosensory stimuli applied to the lumen can release substances from enteroendocrine cells, which could subsequently modulate ENS activity. Advances have been made in optogenetic technologies, such that specific neurochemical classes of enteric neurons can be stimulated. A major focus of this Review will be the latest advances in our understanding of how intrinsic sensory neurons in the ENS detect and respond to sensory stimuli and how these mechanisms differ from extrinsic sensory nerve endings in the gut that underlie the gut-brain axis.

Luminal 5-HT4 receptors-A successful target for prokinetic actions

The prokinetic effects of 5-HT₄ receptor (5-HT₄ R) agonists have been utilized clinically for almost three decades to relieve symptoms of constipation. Surprisingly, the mechanism(s) of action of these compounds is still being debated. Recent studies highlight luminal 5-HT₄ Rs as an alternative and effective target for these prokinetic agents. These include the study by Shokrollahi et al (2019, Neurogastroenterol Motil, e13598) published in the current issue of Neurogastroenterology and Motility, who found that activation of mucosal 5-HT₄ Rs by intraluminal prucalopride, significantly enhanced propulsive motor patterns in rabbit colon. The authors highlight the idea that development of agonists targeting luminal 5-HT₄ Rs in the colonic mucosa might be more effective and safer in achieving prokinetic effects on intestinal motility. The purpose of this mini-review is to discuss the evidence for luminal 5-HT₄ Rs as an emerging target for prokinetic agents in facilitating propulsive motor patterns in the colon.

Deletion of choline acetyltransferase in enteric neurons results in postnatal intestinal dysmotility and dysbiosis

Acetylcholine (ACh)-synthesizing neurons are major components of the enteric nervous system (ENS). They release ACh and peptidergic neurotransmitters onto enteric neurons and muscle. However, pharmacological interrogation has proven inadequate to demonstrate an essential role for ACh. Our objective was to determine whether elimination of ACh synthesis during embryogenesis alters prenatal viability, intestinal function, the neurotransmitter complement, and the microbiome. Conditional deletion of choline acetyltransferase ( ChAT), the ACh synthetic enzyme, in neural crest-derived neurons ( ChAT-Null) was performed. Survival, ChAT activity, gut motility, and the microbiome were studied. ChAT was conditionally deleted in ENS neural crest-derived cells. Despite ChAT absence, mice were born live and survived the first 2 wk. They failed to gain significant weight in the third postnatal week, dying between postnatal d 18 and 30. Small intestinal transit of carmine red was 50% slower in ChAT-Nulls vs. WT and ChAT- Het. The colons of many neonatal ChAT-Null mice contained compacted feces, suggesting dysmotility. Microbiome analysis revealed dysbiosis in ChAT-Null mice. Developmental deletion of ChAT activity in enteric neurons results in proximal gastrointestinal tract dysmotility, critically diminished colonic transit, failure to thrive, intestinal dysbiosis, and death. ACh is necessary for sustained gut motility and survival of neonatal mice after weaning.-Johnson, C. D., Barlow-Anacker, A. J., Pierre, J. F., Touw, K., Erickson, C. S., Furness, J. B., Epstein, M. L., Gosain, A. Deletion of choline acetyltransferase in enteric neurons results in postnatal intestinal dysmotility and dysbiosis.

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.

A1-adenosine acute withdrawal response and cholecystokinin-8 induced contractures are regulated by Ca(2+)- and ATP-activated K(+) channels

In isolated guinea-pig ileum (GPI), the A1-adenosine acute withdrawal response is under the control of several neuronal signalling systems, including the μ/κ-opioid and the cannabinoid CB1 systems. It is now well established that after the stimulation of the A1-adenosine system, the indirect activation of both μ/κ-opioid and CB1 systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated the involvement of the Ca(2+)/ATP-activated K(+) channels in the regulation of both acute A1-withdrawal and CCk-8-induced contractures in the GPI preparation. Interestingly, we found that: (a) the A1-withdrawal contracture is inhibited by voltage dependent Ca(2+)-activated K(+) channels, Kv, while it is enhanced by the voltage independent Ca(2+)-activated K(+) channels, SKCa; (b) in the presence of CCk-8, the inhibitory effect of the A1 agonist, CPA, on the peptide induced contracture is significantly enhanced by the voltage independent Ca(2+)-activated K(+) channel, SKCa; and (c) the A1-withdrawal contracture precipitated in the presence of CCk-8 is controlled by the ATP-sensitive potassium channels, KATP. Our data suggest, for the first time, that both Ca(2+)- and ATP-activated K(+) channels are involved in the regulation of both A1-withdrawal precipitated and CCk-8 induced contractures.

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.

Serotonin signalling in the gut--functions, dysfunctions and therapeutic targets

Serotonin (5-HT) has been recognized for decades as an important signalling molecule in the gut, but it is still revealing its secrets. Novel gastrointestinal functions of 5-HT continue to be discovered, as well as distant actions of gut-derived 5-HT, and we are learning how 5-HT signalling is altered in gastrointestinal disorders. Conventional functions of 5-HT involving intrinsic reflexes include stimulation of propulsive and segmentation motility patterns, epithelial secretion and vasodilation. Activation of extrinsic vagal and spinal afferent fibres results in slowed gastric emptying, pancreatic secretion, satiation, pain and discomfort, as well as nausea and vomiting. Within the gut, 5-HT also exerts nonconventional actions such as promoting inflammation and serving as a trophic factor to promote the development and maintenance of neurons and interstitial cells of Cajal. Platelet 5-HT, originating in the gut, promotes haemostasis, influences bone development and serves many other functions. 5-HT3 receptor antagonists and 5-HT4 receptor agonists have been used to treat functional disorders with diarrhoea or constipation, respectively, and the synthetic enzyme tryptophan hydroxylase has also been targeted. Emerging evidence suggests that exploiting epithelial targets with nonabsorbable serotonergic agents could provide safe and effective therapies. We provide an overview of these serotonergic actions and treatment strategies.

The Nav1.9 channel regulates colonic motility in mice

The colonic migrating motor complex (CMMC) is a major pattern of motility that is entirely generated and organized by the enteric nervous system. We have previously demonstrated that the Na_v1.9 channel underlies a tetrodotoxin-resistant sodium current which modulates the excitability of enteric neurons. The aim of this study was to observe the effect of loss of the Na_v1.9 channel in enteric neurons on mouse colonic motility in vitro. The mechanical activity of the circular muscle was simultaneously recorded from three sites, namely, proximal, mid- and distal, along the whole colon of male, age-matched wild-type and Na_v1.9 null mice. Spontaneous CMMCs were observed in all preparations. The mean frequency of CMMCs was significantly higher in the Na_v1.9 null mice (one every 2.87 ± 0.1 min compared to one every 3.96 ± 0.23 min in the wild type). The mean duration of CMMCs was shorter and the mean area-under-contraction was larger in the Na_v1.9 null mice compared to the wild type. In addition, CMMCs propagated preferentially in an aboral direction in the Na_v1.9 null mice. Our study demonstrates that CMMCs do occur in mice lacking the Na_v1.9 channel, but their characteristics are significantly different from controls. Up to now, the Na_v1.9 channel was mainly associated with nociceptive neurons and involved in their hyperexcitability after inflammation. Our result shows for the first time a role for the Na_v1.9 channel in a complex colonic motor pattern.

Transmission to interneurons is via slow excitatory synaptic potentials mediated by P2Y(1) receptors during descending inhibition in guinea-pig ileum

Background

The nature of synaptic transmission at functionally distinct synapses in intestinal reflex pathways has not been fully identified. In this study, we investigated whether transmission between interneurons in the descending inhibitory pathway is mediated by a purine acting at P2Y receptors to produce slow excitatory synaptic potentials (EPSPs).

Methodology/Principal findings

Myenteric neurons from guinea-pig ileum in vitro were impaled with intracellular microelectrodes. Responses to distension 15 mm oral to the recording site, in a separately perfused stimulation chamber and to electrical stimulation of local nerve trunks were recorded. A subset of neurons, previously identified as nitric oxide synthase immunoreactive descending interneurons, responded to both stimuli with slow EPSPs that were reversibly abolished by a high concentration of PPADS (30 μM, P2 receptor antagonist). When added to the central chamber of a three chambered organ bath, PPADS concentration-dependently depressed transmission through that chamber of descending inhibitory reflexes, measured as inhibitory junction potentials in the circular muscle of the anal chamber. Reflexes evoked by distension in the central chamber were unaffected. A similar depression of transmission was seen when the specific P2Y₁ receptor antagonist MRS 2179 (10 μM) was in the central chamber. Blocking either nicotinic receptors (hexamethonium 200 μM) or 5-HT₃ receptors (granisetron 1 μM) together with P2 receptors had no greater effect than blocking P2 receptors alone.

Conclusions/Significance

Slow EPSPs mediated by P2Y₁ receptors, play a primary role in transmission between descending interneurons of the inhibitory reflexes in the guinea-pig ileum. This is the first demonstration for a primary role of excitatory metabotropic receptors in physiological transmission at a functionally identified synapse.

Serotonin: from top to bottom

Serotonin is a monoamine neurotransmitter, which is phylogenetically conserved in a wide range of species from nematodes to humans. In mammals, age-related changes in serotonin systems are known risk factors of age-related diseases, such as diabetes, faecal incontinence and cardiovascular diseases. A decline in serotonin function with aging would be consistent with observations of age-related changes in behaviours, such as sleep, sexual behaviour and mood all of which are linked to serotonergic function. Despite this little is known about serotonin in relation to aging. This review aims to give a comprehensive analysis of the distribution, function and interactions of serotonin in the brain; gastrointestinal tract; skeletal; vascular and immune systems. It also aims to demonstrate how the function of serotonin is linked to aging and disease pathology in these systems. The regulation of serotonin via microRNAs is also discussed, as are possible applications of serotonergic drugs in aging research and age-related diseases. Furthermore, this review demonstrates that serotonin is potentially involved in whole organism aging through its links with multiple organs, the immune system and microRNA regulation. Methods to investigate these links are discussed.

Serotonin-induced contraction in isolated intestine from a teleost fish (Carassius auratus): characterization and interactions with melatonin

BACKGROUND

Serotonin (5-HT) plays a critical role in several gastrointestinal functions in vertebrates. In teleosts lacking enterochromaffin cells, intestinal 5-HT originates from serotonergic enteric neurons. In the present study, the foregut of a stomachless teleost, the goldfish (Carassius auratus), was used to evaluate the in vitro effect of 5-HT on fish intestinal motility. We also studied the role of melatonin (MEL), an indoleamine sharing the biosynthetic pathway with 5-HT, as regulator of serotonergic activity.

METHODS

An organ bath system, with longitudinal strips from the goldfish intestinal bulb attached to an isometric transducer was used to record foregut smooth muscle contractions.

KEY RESULTS

Concentration-dependent curves of the contractile response exerted by 5-HT and its agonists, 5-methoxytryptamine (5-MT) and 5-carboxamidotryptamine (5-CT), suggest a receptor-mediated action, supported by the blockade by a general 5-HT antagonist, methysergide. The 5-HT-induced contraction was abolished in the presence of atropine, revealing the involvement of cholinergic transmission in gut actions of 5-HT. Furthermore, MEL inhibited the contractile effect of 5-HT and its agonists by up to 50%, which was counteracted by MEL antagonists.

CONCLUSIONS & INFERENCES

We can provisionally propose that at least two different 5-HT receptor subtypes are involved in fish intestinal motility, a 5-HT₄-like (5-MT-preferring) and a 5-HT₇-like (5-CT- and fluphenazine-sensitive) receptor. In summary, our results indicate that 5-HT regulates the contractile activity of goldfish foregut through specific receptors located in cholinergic neurons, and that MEL can modulate these serotonergic actions through high-affinity membrane receptors.

Critical role of 5-HT1A, 5-HT3, and 5-HT7 receptor subtypes in the initiation, generation, and propagation of the murine colonic migrating motor complex

The colonic migrating motor complex (CMMC) is necessary for fecal pellet propulsion in the murine colon. We have previously shown that 5-hydroxytryptamine (5-HT) released from enterochromaffin cells activates 5-HT(3) receptors on the mucosal processes of myenteric Dogiel type II neurons to initiate the events underlying the CMMC. Our aims were to further investigate the roles of 5-HT(1A), 5-HT(3), and 5-HT(7) receptor subtypes in generating and propagating the CMMC using intracellular microelectrodes or tension recordings from the circular muscle (CM) in preparations with and without the mucosa. Spontaneous CMMCs were recorded from the CM in isolated murine colons but not in preparations without the mucosa. In mucosaless preparations, ondansetron (3 microM; 5-HT(3) antagonist) plus hexamethonium (100 microM) completely blocked spontaneous inhibitory junction potentials, depolarized the CM. Ondansetron blocked the preceding hyperpolarization associated with a CMMC. Spontaneous CMMCs and CMMCs evoked by spritzing 5-HT (10 and 100 microM) or nerve stimulation in preparations without the mucosa were blocked by SB 258719 or SB 269970 (1-5 microM; 5-HT(7) antagonists). Both NAN-190 and (S)-WAY100135 (1-5 microM; 5-HT(1A) antagonists) blocked spontaneous CMMCs and neurally evoked CMMCs in preparations without the mucosa. Both NAN-190 and (S)-WAY100135 caused an atropine-sensitive depolarization of the CM. The precursor of 5-HT, 5-hydroxytryptophan (5-HTP) (10 microM), and 5-carboxamidotryptamine (5-CT) (5 microM; 5-HT(1/5/7) agonist) increased the frequency of spontaneous CMMCs. 5-HTP and 5-CT also induced CMMCs in preparations with and without the mucosa, which were blocked by SB 258719. 5-HT(1A), 5-HT(3), and 5-HT(7) receptors, most likely on Dogiel Type II/AH neurons, are important in initiating, generating, and propagating the CMMC. Tonic inhibition of the CM appears to be driven by ongoing activity in descending serotonergic interneurons; by activating 5-HT(7) receptors on AH neurons these interneurons also contribute to the generation of the CMMC.

Synaptic transmission at functionally identified synapses in the enteric nervous system: roles for both ionotropic and metabotropic receptors

Digestion and absorption of nutrients and the secretion and reabsorption of fluid in the gastrointestinal tract are regulated by neurons of the enteric nervous system (ENS), the extensive peripheral nerve network contained within the intestinal wall. The ENS is an important physiological model for the study of neural networks since it is both complex and accessible. At least 20 different neurochemically and functionally distinct classes of enteric neurons have been identified in the guinea pig ileum. These neurons express a wide range of ionotropic and metabotropic receptors. Synaptic potentials mediated by ionotropic receptors such as the nicotinic acetylcholine receptor, P2X purinoceptors and 5-HT(3) receptors are seen in many enteric neurons. However, prominent synaptic potentials mediated by metabotropic receptors, like the P2Y(1) receptor and the NK(1) receptor, are also seen in these neurons. Studies of synaptic transmission between the different neuron classes within the enteric neural pathways have shown that both ionotropic and metabotropic synaptic potentials play major roles at distinct synapses within simple reflex pathways. However, there are still functional synapses at which no known transmitter or receptor has been identified. This review describes the identified roles for both ionotropic and metabotropic neurotransmission at functionally defined synapses within the guinea pig ileum ENS. It is concluded that metabotropic synaptic potentials act as primary transmitters at some synapses. It is suggested identification of the interactions between different synaptic potentials in the production of complex behaviours will require the use of well validated computer models of the enteric neural circuitry.

The neurochemistry and innervation patterns of extrinsic sensory and sympathetic nerves in the myenteric plexus of the C57Bl6 mouse jejunum

In vitro anterograde tracing of axons in mesenteric nerve trunks using biotinamide in combination with immunohistochemical labelling was used to characterize the extrinsic nerve projections in the myenteric plexus of the mouse jejunum. Anterogradely-labelled spinal sensory fibres innervating the enteric nervous system were identified by their immunoreactivity for calcitonin gene-related peptide (CGRP), while sympathetic noradrenergic fibres were detected with tyrosine hydroxylase (TH), using confocal microscopy. The presence of these markers has been previously described in the spinal sensory and sympathetic fibres. Labelled extrinsic nerve fibres in the myenteric plexus were identified apposing enteric neurons that were immunoreactive for either calretinin (CalR), calbindin (CalB) or nitric oxide synthase (NOS). Of the total anterogradely labelled axons in the myenteric plexus, 20% were CGRP-immunoreactive. Labelled CGRP-immunoreactive varicosities were closely apposed to CalR-immunoreactive myenteric cells, many of which were Dogiel type I (40%; interneurons) or type II (20%; intrinsic sensory) neurons. Labelled CGRP-immunoreactive varicosities were also observed in close appositions to CalB-immunoreactive myenteric cell bodies, of which a small subset had type II morphology (18%; intrinsic sensory neurons). A further 43% of all biotinamide-filled fibres were immunoreactive for TH and these fibres were apposed to CalR-immunoreactive cell bodies (small-sized; excitatory motor neurons) and NOS-immunoreactive cell bodies (either type I or small neurons; inhibitory motor neurons and interneurons) in the myenteric plexus. The results provide a neurochemical and neuroanatomical basis for connections between dorsal root afferent neurons and myenteric neurons and suggest an anatomical substrate for the well-known modulation of enteric circuits from sympathetic nerves. No anterogradely-labelled fibres were stained for NOS-immunoreactivity, despite more than 60% of dorsal root ganglion (DRG) neurons retrogradely labelled from the jejunum showing NOS-immunoreactivity. This was due to a substantial, time-dependent, and apparently selective, loss of NOS from extrinsic axons under in vitro conditions. Lastly, a small population of non-immunoreactive biotinamide-filled fibres (<1%) gave rise to dense terminal structures around individual myenteric cell bodies lacking CalR, CalB or NOS. These specialized endings may represent vagal fibres or a subset of spinal sensory neurons that do not contain CGRP.

The cornucopia of intestinal chemosensory transduction

The chemosensory transduction mechanisms that the gastrointestinal (GI) tract uses to detect chemical and nutrient stimuli are poorly understood. The GI tract is presented with a wide variety of stimuli including potentially harmful chemicals or toxins as well as 'normal' stimuli including nutrients, bacteria and mechanical forces. Sensory transduction is at its simplest the conversion of these stimuli into a neural code in afferent nerves. Much of the information encoded is used by the enteric nervous system to generate local reflexes while complementary information is sent to the central nervous system via afferents or by release of hormones to affect behaviour. This review focuses on the chemosensory transduction mechanisms present in the GI tract. It examines the expression and localisation of the machinery for chemosensory transduction. It summarises the types of cells which might be involved in detecting stimuli and releasing neuroactive transmitters. Finally, it highlights the idea that chemosensory transduction mechanisms in the GI tract utilise many overlapping and complementary mechanisms for detecting and transducing stimuli into reflex action.

Calcium activity in different classes of myenteric neurons underlying the migrating motor complex in the murine colon

The spontaneous colonic migrating motor complex (CMMC) is a cyclical contractile and electrical event that is the primary motor pattern underlying fecal pellet propulsion along the murine colon. We have combined Ca(2+) imaging with immunohistochemistry to determine the role of different classes of myenteric neurons during the CMMC. Between CMMCs, myenteric neurons usually displayed ongoing but uncoordinated activity. Stroking the mucosa at the oral or anal end of the colon resulted in a CMMC (latency: 6 to 10 s; duration: 28 s) that consisted of prolonged increases in activity in many myenteric neurons that was correlated to Ca(2+) transients in and displacement of the muscle. These neurons were likely excitatory motor neurons. Activity in individual neurons during the CMMC was similar regardless of whether the CMMC occurred spontaneously or was evoked by anal or oral mucosal stimulation. This suggests that convergent interneuronal pathways exist which generate CMMCs. Interestingly, Ca(2+) transients in a subset of NOS +ve neurons were substantially reduced during the CMMC. These neurons are likely to be inhibitory motor neurons that reduce their activity during a complex (disinhibition) to allow full excitation of the muscle. Local stimulation of the mucosa evoked synchronized Ca(2+) transients in Dogiel Type II (mitotracker/calbindin-positive) neurons after a short delay (1-2 s), indicating they were the sensory neurons underlying the CMMC. These local responses were observed in hexamethonium, but were blocked by ondansetron (5-HT(3) antagonist), suggesting Dogiel Type II neurons were activated by 5-HT release from enterochromaffin cells in the mucosa. In fact, removal of the mucosa yielded no spontaneous CMMCs, although many neurons (NOS +ve and NOS ve) exhibited ongoing activity, including Dogiel Type II neurons. These results suggest that spontaneous or evoked 5-HT release from the mucosa is necessary for the activation of Dogiel Type II neurons that generate CMMCs.

5-HT antagonists NAN-190 and SB 269970 block alpha2-adrenoceptors in the guinea pig

Serotonin (5-HT) plays a significant role in the regulation of intestinal secretion of water and electrolytes. The initial aim of this study was to use intracellular recording and specific antagonists to identify roles of 5-HT1A and 5-HT7 receptors of submucosal noncholinergic secretomotor neurons of guinea pig ileum, in vitro. However, it was found that the widely used 5-HT receptor antagonists NAN-190 (5-HT1A) and SB 269970 (5-HT7) both blocked alpha2-adrenoceptors, and hence depressed inhibitory synaptic potentials and hyperpolarizations evoked by noradrenaline, in these neurons. Both compounds enhanced neurally evoked contractions of the guinea pig vas deferens, an effect characteristic of blockade of alpha2-adrenoceptors. These results raise significant concerns about studies using NAN-190 and SB 269970 as specific antagonists of serotonin receptors.

Serotonin release and uptake in the gastrointestinal tract

The afferent innervation of the gastrointestinal (GI) tract consists of intrinsic and extrinsic sensory neurons that respond to nutrients, chemicals or mechanical stimuli within the gut lumen. Most stimuli do not interact directly with the afferent nerves but instead activate specialised cells in the epithelium in a process of sensory transduction. It is thought that one of the first steps in this process is the release of serotonin (5-HT) from the enterochromaffin (EC) cells. The EC cells are a sub-type of enteroendocrine (EE) cells which are found among the enterocytes of the intestinal epithelium. The EC cells are responsible for the production and storage of the largest pool of 5 HT in the body. Released 5-HT can act on the intrinsic nerves and vagal endings. This review will focus on the role of 5-HT in sensory transduction and examine how the EC cell produces and releases 5-HT. We will explore recent developments that have helped to elucidate some of the proteins that allow EC cells to sense the luminal environment. Finally, we will highlight some of the findings from new studies using electrochemical techniques which allow the real-time recording of 5-HT concentrations near to the EC cell.

Slow synaptic transmission in myenteric AH neurons from the inflamed guinea pig ileum

We investigated the effect of inflammation on slow synaptic transmission in myenteric neurons in the guinea pig ileum. Inflammation was induced by the intraluminal injection of trinitrobenzene sulfonate, and tissues were taken for in vitro investigation 6-7 days later. Brief tetanic stimulation of synaptic inputs (20 Hz, 1 s) induced slow excitatory postsynaptic potentials (EPSPs) in 49% and maintained postsynaptic excitation that lasted from 27 min to 3 h in 13% of neurons from the inflamed ileum. These neurons were classified electrophysiologically as AH neurons; 10 were morphological type II neurons, and one was type I. Such long-term hyperexcitability after a brief stimulus is not encountered in enteric neurons of normal intestine. Electrophysiological properties of neurons with maintained postsynaptic excitation were similar to those of neurons with slow EPSPs. Another form of prolonged excitation, sustained slow postsynaptic excitation (SSPE), induced by 1-Hz, 4-min stimulation, in type II neurons from the inflamed ileum reached its peak earlier but had lower amplitude than that in control. Unlike slow EPSPs and similar to SSPEs, maintained excitation was not inhibited by neurokinin-1 or neurokinin-3 receptor antagonists. Maintained postsynaptic excitation was not influenced by PKC inhibitors, but the PKA inhibitor, H-89, caused further increase in neuronal excitability. In conclusion, maintained excitation, observed only in neurons from the inflamed ileum, may contribute to the dysmotility, pain, and discomfort associated with intestinal inflammation.

Electrical stimulation of the mucosa evokes slow EPSPs mediated by NK1 tachykinin receptors and by P2Y1 purinoceptors in different myenteric neurons

Slow excitatory postsynaptic potentials (EPSPs) in enteric neurons arise from diverse sources, but which neurotransmitters mediate specific types of slow EPSPs is unclear. We investigated transmitters and receptors mediating slow EPSPs in myenteric neurons evoked by electrical stimulation of the mucosa in guinea pig small intestine. Segments of ileum or jejunum were dissected to allow access to the myenteric plexus adjacent to intact mucosa, in vitro. AH and S neurons were impaled with conventional intracellular electrodes. Trains of stimuli delivered to the mucosa evoked slow EPSPs in AH neurons that were blocked or depressed by the neurokinin-1 (NK1) tachykinin antagonist SR140333 (100 nM) in 10 of 11 neurons; the NK3 tachykinin receptor antagonist SR142801 (100 nM) had no effect on slow EPSPs in seven of nine AH neurons. Single pulses to the mucosa evoked fast EPSPs and slow depolarizations in S neurons. The depolarizations were divided into intermediate (durations 300-900 ms) or slow (durations 1.3-9 s) EPSPs. The slow EPSPs were blocked by pyridoxal phosphate-6-axophenyl-2-4-disulfonic acid (30 microM, N = 3) or the specific P2Y(1) antagonist MRS 2179 (10 microM, N = 6) and were predominantly in anally projecting S neurons that were immunoreactive for nitric oxide synthase (NOS). In contrast, intermediate EPSPs were predominantly evoked in NOS-negative neurons; these were abolished by MRS 2179 (N = 8). Thus activation of pathways running from the mucosa excites three different types of slow EPSP in myenteric neurons, which are mediated by either a tachykinin (NK1, AH neurons) or a purine nucleotide (P2Y(1), S neurons).

5-hydroxyindalpine, an agonist at the putative 5-HT receptor, has no activity on human recombinant monoamine receptors but accelerates distension-induced peristalsis in mouse isolated colon

Although the putative 5-HT(1P) receptor has been implicated to have a role in peristalsis, experiments which suggest this function are preliminary or have measured only components of the reflex. We have, therefore, further characterized a reported agonist at this receptor (5-hydroxyindalpine; 5-OHIP) and investigated the effects of 5-OHIP and 5-hydroxytrytophan-dipeptide (5-HTP-DP), a reported 5-HT(1P) receptor antagonist, on distension-induced peristalsis in mouse colon. The effects of 5-OHIP on intracellular calcium, cyclic adenosine monophosphate concentrations or GTPgammaS binding were measured in cell lines expressing human recombinant 5-HT(1A, 1B, 1D, 2A, 2B, 2C, 3, 4, 6, 7) and alpha(1A), alpha(1B), D(1), D(2), D(3), H(1), H(3) receptors. The effects of 5-OHIP and 5-HTP-DP on peristalsis were assessed by measuring changes in frequency and times to reach threshold of peristaltic contractions, as well as the threshold and maximum pressures of each peristaltic stroke. 5-hydroxyindalpine (1 nmol L(-1)-10 micromol L(-1)) had no significant activity at any of the receptors studied. However, 5-OHIP (0.1 nmol L(-1)-1 micromol L(-1)) concentration-dependently increased the frequency of peristalsis (EC(50) = 4.4 nmol L(-1)) and reduced the time taken to reach threshold and threshold pressure, without altering maximum pressures. The maximum effect of 5-OHIP was at 1 micromol L(-1) (68.0 +/- 14.5% increase in frequency); 10 micromol L(-1) decreased peristalsis. 5-hydroxytrytophan-dipeptide (1-300 nmol L(-1)) also increased the frequency of peristalsis and prevented the actions of 5-OHIP. The higher concentration (1 micromol L(-1)) transiently inhibited peristalsis and after recovery, prevented the actions of 5-OHIP but not the excitatory activity of the cholinesterase inhibitor neostigmine. In summary, the present data demonstrate an interaction of '5-HT(1P)-ligands' with the peristaltic reflex. However, the absence of an effect of 5-OHIP on a range of different monoamine receptors continues to highlight the need to investigate the identity of the putative 5-HT(1P) receptor.

Modulatory effects of serotonin on GABAergic synaptic transmission and membrane properties in the deep cerebellar nuclei

Cerebellar outputs from the deep cerebellar nuclei (DCN) are critical for generating and controlling movement. DCN neuronal activity is primarily controlled by GABAergic inhibitory transmission by Purkinje cells in the cerebellar cortex and is also modulated by nerve inputs originating from other brain regions within and outside the cerebellum. In this study, we examined the modulatory effects of 5-HT on GABAergic synapses in the DCN. 5-HT decreased the amplitude of stimulation-evoked inhibitory postsynaptic currents (eIPSCs) in DCN neurons, and this effect was abolished by a 5-HT(1B) antagonist, SB 224289. The decrease in IPSC amplitude was associated with an increased paired-pulse ratio of the IPSC. 5-HT also decreased the frequency of miniature IPSCs without altering the amplitude. These data suggest that 5-HT presynaptically inhibited GABA release. Furthermore, 5-HT elicited a slow inward current in DCN neurons. Pharmacological studies showed that 5-HT activated the 5-HT(5) receptor, which is positively coupled to G protein and elicited the slow inward current through enhancement of hyperpolarization-activated cation channel activation. Finally, we examined the effects of 5-HT on the spike generation that accompanies repetitive stimulation of inhibitory synapses. 5-HT increased the spontaneous firing rate in DCN neurons caused by depolarization. Increase in the 5-HT-induced tonic firing relatively decreased the contrast difference from the rebound depolarization-induced firing. However, the inhibitory transmission-induced silencing of DCN firing remained during the conditioning stimulus. These results suggest that 5-HT plays a regulatory role in spike generation and contributes to the gain control of inhibitory GABAergic synapses in DCN neurons.

Targets of myenteric interneurons in the guinea-pig small intestine

Polarized outputs of myenteric interneurons in guinea-pig small intestine have been well studied. However, the variety of motility patterns exhibited suggests that some interneuron targets remain unknown. We used antisera selected to distinguish interneuron varicosities and known myenteric neuron types to investigate outputs of three interneuron classes in guinea-pig jejunum; two classes of descending interneurons immunoreactive (IR) for somatostatin (SOM) or nitric oxide synthase (NOS)/vasoactive intestinal peptide (VIP), and one class of ascending interneurons [calretinin/enkephalin (ENK)-IR]. Varicosities apposed to immunohistochemically identified cell bodies were quantified by confocal microscopy. Intrinsic sensory neurons (calbindin-IR) were apposed by few varicosities. Cholinergic secretomotor neurons (neuropeptide Y-IR) were apposed by many SOM-IR varicosities. Longitudinal muscle excitatory motor neurons (calretinin-IR) were apposed by some VIP- and ENK-IR varicosities, but few SOM-IR varicosities. Ascending interneurons (calretinin-IR) were apposed by many varicosities of all types. NOS-IR interneurons and inhibitory motor neurons were apposed by numerous VIP-IR and SOM-IR varicosities. NOS-IR short inhibitory motor neurons were apposed by significantly fewer ENK-IR varicosities than other NOS-IR neurons. Based on the specific chemical coding of ascending (ENK) and descending (SOM) interneurons, we conclude that cholinergic secretomotor neurons and short inhibitory neurons are located in descending reflex pathways, while ascending interneurons and NOS-IR descending interneurons are focal points at which ascending and descending pathways converge.

Serotonin pharmacology in the gastrointestinal tract: a review

Serotonin (5-hydroxytryptamine or 5-HT) plays a critical physiological role in the regulation of gastrointestinal (GI) function. 5-HT dysfunction may also be involved in the pathophysiology of a number of functional GI disorders, such as chronic constipation, irritable bowel syndrome and functional dyspepsia. This article describes the role of 5-HT in the enteric nervous system (ENS) of the mammalian GI tract and the receptors with which it interacts. Existing serotonergic therapies that have proven effective in the treatment of GI functional disorders and the potential of drugs currently in development are also highlighted. Advances in our understanding of the physiological and pathophysiological roles of 5-HT in the ENS and the identification of selective receptor ligands bodes well for the future development of more efficacious therapies for patients with functional GI disorders.

Mechanisms of serotonergic agents for treatment of gastrointestinal motility and functional bowel disorders

Most of the body's serotonin is released in the gut where it plays an important role in the control of gastrointestinal (GI) motility, sensitivity and muscle tone by activating different receptor subtypes. This review focuses on the known effects of selective serotonin reuptake inhibitor and serotonin receptor agonists and antagonists on the sensorimotor function of the GI tract and describes the therapeutic potential of these actions for GI motility and functional bowel disorders.

Neuropathophysiology of functional gastrointestinal disorders

The investigative evidence and emerging concepts in neurogastroenterology implicate dysfunctions at the levels of the enteric and central nervous systems as underlying causes of the prominent symptoms of many of the functional gastrointestinal disorders. Neurogastroenterological research aims for improved understanding of the physiology and pathophysiology of the digestive subsystems from which the arrays of functional symptoms emerge. The key subsystems for defecation-related symptoms and visceral hyper-sensitivity are the intestinal secretory glands, the musculature and the nervous system that controls and integrates their activity. Abdominal pain and discomfort arising from these systems adds the dimension of sensory neurophysiology. This review details current concepts for the underlying pathophysiology in terms of the physiology of intestinal secretion, motility, nervous control, sensing function, immuno-neural communication and the brain-gut axis.

Mapping 5-HT inputs to enteric neurons of the guinea-pig small intestine

5-HT released by gastrointestinal mucosa and enteric interneurons has powerful effects on gut behavior. However, the targets of 5-HT-containing neurons within enteric circuits are not well characterized. We used antisera against 5-HT and selected markers of known enteric neuron types to investigate the connections made by 5-HT-containing neurons in the guinea-pig jejunum. Confocal microscopy was used to quantify the number of 5-HT-immunoreactive varicosities apposed to immunohistochemically identified cell bodies. Large numbers of varicosities were identified apposing cholinergic secretomotor neurons, immunoreactive for neuropeptide Y, in both myenteric and submucous plexuses. Subgroups of neurons identified by calretinin (ascending interneurons) and nitric oxide synthase (descending interneurons and inhibitory motor neurons) immunoreactivity were also apposed by many varicosities. Longitudinal muscle motor neurons (calretinin immunoreactive) and AH/Dogiel type II (sensory) neurons (calbindin immunoreactive) were apposed by small numbers of varicosities. Combined retrograde tracing and immunohistochemistry were used to identify excitatory circular muscle motor neurons; these were encircled by 5-HT-immunoreactive varicosities, but the appositions could not be quantified. We suggest that 5-HT-containing interneurons are involved in secretomotor pathways and pathways to subgroups of other interneurons, but not longitudinal muscle motor neurons. There also appear to be connections between 5-HT-containing interneurons and excitatory circular muscle motor neurons. Physiological evidence demonstrates a functional connection between 5-HT-containing interneurons and AH/Dogiel type II neurons, but few 5-HT-immunoreactive varicosities were observed apposing calbindin-immunoreactive cell bodies. Taken together these results suggest that neural 5-HT may have significant roles in excitatory pathways regulating both motility and secretion.

Serotonergic and non-serotonergic targets in the pharmacotherapy of visceral hypersensitivity

Visceral hypersensitivity is considered a key mechanism in the pathogenesis of functional gastrointestinal (GI) disorders. Targeting visceral hypersensitivity seems an attractive approach to the development of drugs for functional GI disorders. This review summarizes current knowledge on targets for the treatment of visceral hypersensitivity, and the status of current and future drug and probiotic treatment development, and the role of pharmacogenomic factors.

Sumatriptan is an agonist at 5-HT receptors on myenteric neurones in the guinea-pig gastric antrum

Sumatriptan, a 5-hydroxytryptamine(1D) (5-HT(1D))-receptor agonist used in the treatment in migraine, inhibits gastric motility via the enteric nervous system. As no studies have reported enteric neuronal 5-HT(1D) receptors, we used conventional intracellular recordings to characterize the actions of sumatriptan on 145 guinea-pig antral myenteric neurones. In 24 of 29 neurones with a 5-HT(1P) receptor-mediated depolarizing response to 5-HT, application of sumatriptan caused a dose-dependent depolarization, accompanied by increased membrane resistance and enhanced excitability. Depolarizing responses to sumatriptan occurred both in cholinergic and in nitrergic neurones. Sumatriptan did not mimic the 5-HT(3) receptor-mediated fast-depolarizing responses or 5-HT(1A) receptor-mediated inhibitory responses to 5-HT. Sumatriptan had no effect on neurones not responding to 5-HT. The depolarizing response to sumatriptan was inhibited by renzapride, but not by 5-HT(1-7) receptor antagonists. We conclude that sumatriptan behaves as an agonist at the 5-HT(1P) receptor on myenteric neurones in the guinea-pig gastric antrum. The actions of sumatriptan on gastric motility seem to be attributable to a direct action on enteric neurones.

The serotonin signaling system: from basic understanding to drug development for functional GI disorders

Serotonin is an important gastrointestinal signaling molecule. It is a paracrine messenger utilized by enterochromaffin (EC) cells, which function as sensory transducers. Serotonin activates intrinsic and extrinsic primary afferent neurons to, respectively, initiate peristaltic and secretory reflexes and to transmit information to the central nervous system. Serotonin is also a neurotransmitter utilized by a system of long descending myenteric interneurons. Serotonin is synthesized through the actions of 2 different tryptophan hydroxylases, TpH1 and TpH2, which are found, respectively, in EC cells and neurons. Serotonin is inactivated by the serotonin reuptake transporter (SERT)-mediated uptake into enterocytes or neurons. The presence of many serotonin receptor subtypes enables selective drugs to be designed to therapeutically modulate gastrointestinal motility, secretion, and sensation. Current examples include tegaserod, a 5-HT(4) partial agonist, which has been approved for treatment of irritable bowel syndrome (IBS) with constipation in women and for chronic constipation in men and women. The 5-HT(3) antagonists, granisetron and ondansetron, are useful in combating the nausea associated with cancer chemotherapy, and alosetron is employed in the treatment of IBS with diarrhea. Serotonergic signaling abnormalities have also been putatively implicated in the pathogenesis of functional bowel diseases. Other compounds, for which efficacy has not been rigorously established, but which may have value, include tricyclic antidepressants and serotonin selective reuptake inhibitors to combat IBS, and 5-HT(1) agonists, which enhance gastric accommodation, to treat functional dyspepsia. The initial success encountered with serotonergic agents holds promise for newer and more potent insights and therapies of brain-gut disorders.

Fundamentals of neurogastroenterology: basic science

The focus of neurogastroenterology in Rome II was the enteric nervous system (ENS). To avoid duplication with Rome II, only advances in ENS neurobiology after Rome II are reviewed together with stronger emphasis on interactions of the brain, spinal cord, and the gut in terms of relevance for abdominal pain and disordered gastrointestinal function. A committee with expertise in selective aspects of neurogastroenterology was invited to evaluate the literature and provide a consensus overview of the Fundamentals of Neurogastroenterology textbook as they relate to functional gastrointestinal disorders (FGIDs). This review is an abbreviated version of a fuller account that appears in the forthcoming book, Rome III. This report reviews current basic science understanding of visceral sensation and its modulation by inflammation and stress and advances in the neurophysiology of the ENS. Many of the concepts are derived from animal studies in which the physiologic mechanisms underlying visceral sensitivity and neural control of motility, secretion, and blood flow are examined. Impact of inflammation and stress in experimental models relative to FGIDs is reviewed as is human brain imaging, which provides a means for translating basic science to understanding FGID symptoms. Investigative evidence and emerging concepts implicate dysfunction in the nervous system as a significant factor underlying patient symptoms in FGIDs. Continued focus on neurogastroenterologic factors that underlie the development of symptoms will lead to mechanistic understanding that is expected to directly benefit the large contingent of patients and care-givers who deal with FGIDs.

Enteric nervous system

PURPOSE OF REVIEW

Our aim was to provide a synopsis of how the field of enteric neurobiology has advanced during the past year.

RECENT FINDINGS

With such a large number of studies to choose from and given our emphasis in last year's issue on developmental aspects of the enteric nervous system, we have focused on several key themes reflecting the current interest in the way the enteric nervous system is altered in disease.

SUMMARY

The new basic science information gathered during the past year provides insight into pathophysiological processes and will pave the way for improved understanding of both organic and 'functional' gastrointestinal disorders.

5-HT7 receptors modulate peristalsis and accommodation in the guinea pig ileum

BACKGROUND & AIMS

The 5-hydroxytryptamine 7 (5-HT7) receptors mediate intestinal smooth muscle relaxation. In this study, we evaluated the expression of 5-HT7 receptors in the guinea pig ileum and their role in peristalsis and accommodation of the circular muscle.

METHODS

We used immunohistochemistry and confocal microscopy with whole tissue and cultured myenteric neurons. Peristalsis was induced by delivering a solution into the oral end of an isolated ileal segment. The effect of the selective 5-HT7 receptor antagonist SB-269970 (100 nmol/L) on peristaltic activity was evaluated at 30, 60, and 90 minutes and compared with control.

RESULTS

5-HT7 receptor immunoreactivity was localized to numerous myenteric neurons, a few submucosal neurons, and a few smooth muscle cells of the ileum. In enteric cultured neurons, 5-HT7 receptor immunoreactivity was observed in subpopulations of after hyperpolarizing neurons and descending neurons as identified by neuron-specific nuclear protein or calbindin and neuronal nitric oxide synthase or vasoactive intestinal peptide antibodies, respectively. SB-269970 significantly increased the threshold pressure by 33.3% +/- 2.2% (P < .001) and by 27.2% +/- 1.6% (P < .05) at 60 and 90 minutes, respectively, without modifying the threshold volume. The accommodation significantly decreased by 27.5% both at 60 and 90 minutes (P < .05).

CONCLUSIONS

Our results indicate that endogenous 5-HT is involved in the modulation of circular muscle accommodation during the preparatory phase of peristalsis via the activation of 5-HT7 receptors expressed by neurons in addition to smooth muscle cells. Overstimulation of these receptors leading to an exaggerated accommodation of circular muscle might contribute to abdominal symptoms in functional bowel disorders.