Classes of enteric nerve cells in the guinea-pig small intestine

Insights into the Role of Opioid Receptors in the GI Tract: Experimental Evidence and Therapeutic Relevance

Opioid drugs are prescribed extensively for pain treatment but when used chronically they induce constipation that can progress to opioid-induced bowel dysfunction. Opioid drugs interact with three classes of opioid receptors: mu opioid receptors (MORs), delta opioid receptors (DOR), and kappa opioid receptors (KORs), but opioid drugs mostly target the MORs. Upon stimulation, opioid receptors couple to inhibitory Gi/Go proteins that activate or inhibit downstream effector proteins. MOR and DOR couple to inhibition of adenylate cyclase and voltage-gated Ca2+ channels and to activation of K+ channels resulting in reduced neuronal activity and neurotransmitter release. KORs couple to inhibition of Ca2+ channels and neurotransmitter release. In the gastrointestinal tract, opioid receptors are localized to enteric neurons, interstitial cells of Cajal, and immune cells. In humans, MOR, DOR, and KOR link to inhibition of acetylcholine release from enteric interneurons and motor neurons and purine/nitric oxide release from inhibitory motor neurons causing inhibition of propulsive motility patterns. MOR and DOR activation also results in inhibition of submucosal secretomotor neurons reducing active Cl- secretion and passive water movement into the colonic lumen. Together, these effects on motility and secretion account for the constipation caused by opioid receptor agonists. Tolerance develops to the analgesic effects of opioid receptor agonists but not to the constipating actions. This may be due to differences in trafficking and downstream signaling in enteric nerves in the colon compared to the small intestine and in neuronal pain pathways. Further studies of differential opioid receptor desensitization and tolerance in subsets of enteric neurons may identify new drug or other treatment strategies of opioid-induced bowel dysfunction.

Stress, sex, and the enteric nervous system

Made up of millions of enteric neurons and glial cells, the enteric nervous system (ENS) is in a key position to modulate the secretomotor function and visceral pain of the gastrointestinal tract. The early life developmental period, through which most of the ENS development occurs, is highly susceptible to microenvironmental perturbation. Over the past decade, accumulating evidence has shown the impact of stress and early life adversity (ELA) on host gastrointestinal pathophysiology. While most of the focus has been on alterations in brain structure and function, limited experimental work in rodents suggest that the enteric nervous system can also be directly affected, as shown by changes in the number, phenotype, and reactivity of enteric nerves. The work of Medland et al. in the current issue of this journal demonstrates that such alterations also occur in pigs, a larger mammalian species with high translational value to human. This work also highlights a sex-differential susceptibility of the ENS to the effect of ELA, which could contribute to the higher prevalence of GI disorders in women. In this mini-review, we will discuss the development and composition of the ENS and related gastrointestinal sensory motor and secretory functions. We will then focus on the influence of stress on the enteric nervous system, with a particular emphasis on neurodevelopmental changes. Finally, we will discuss the influence of sex on those parameters.

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

The Role of Central and Enteric Nervous Systems in the Control of the Retrograde Giant Contraction

Background/Aims

The role of the enteric (ENS) and central (CNS) nervous systems in the control of the retrograde giant contraction (RGC) associated with vomiting is unknown.

Methods

The effects of myotomy or mesenteric nerve transection (MNT) on apomorphine-induced emesis were investigated in 18 chronically instrumented dogs

Results

Neither surgery affected the RGC orad of the surgical site or the velocity of the RGC over the entire small intestine. Myotomy blocked the RGC for 17 ± 5 cm aborad of the myotomy, and the velocity of the RGC from 100 to 70 cm from the pylorus slowed (18.1 ± 3.0 to 9.0 ± 0.8 cm/sec) such that the RGC orad and aborad of the myotomy occurred simultaneously. After MNT, the RGC was unchanged up to 66 ± 6 cm from the pylorus, and the sequence of the RGC across the denervated intestine was unaltered. The velocity of the RGC from 100 to 70 cm from the pylorus increased from 12.8 ± 1.6 to 196 ± 116 cm/sec. After myotomy or MNT, the percent occurrence and magnitude of the RGC across the intestine 100 to 70 cm from the pylorus decreased.

Conclusions

The CNS activates the RGC 10 to 20 cm aborad of its innervation of the intestine and controls the RGC sequence. On the other hand, the ENS plays a role in initiation and generation of the RGC.

Neuromechanical factors involved in the formation and propulsion of fecal pellets in the guinea-pig colon

BACKGROUND

The neuromechanical processes involved in the formation and propulsion of fecal pellets remain incompletely understood.

METHODS

We analyzed motor patterns in isolated segments of the guinea-pig proximal and distal colon, using video imaging, during oral infusion of liquid, viscous material, or solid pellets.

KEY RESULTS

Colonic migrating motor complexes (CMMCs) in the proximal colon divided liquid or natural semisolid contents into elongated shallow boluses. At the colonic flexure these boluses were formed into shorter, pellet-shaped boluses. In the non-distended distal colon, spontaneous CMMCs produced small dilations. Both high- and low-viscosity infusions evoked a distinct motor pattern that produced pellet-shaped boluses. These were propelled at speeds proportional to their surface area. Solid pellets were propelled at a speed that increased with diameter, to a maximum that matched the diameter of natural pellets. Pellet speed was reduced by increasing resistive load. Tetrodotoxin blocked all propulsion. Hexamethonium blocked normal motor patterns, leaving irregular propagating contractions, indicating the existence of neural pathways that did not require nicotinic transmission.

CONCLUSIONS & INFERENCES

Colonic migrating motor complexes are responsible for the slow propulsion of the soft fecal content in the proximal colon, while the formation of pellets at the colonic flexure involves a content-dependent mechanism in combination with content-independent spontaneous CMMCs. Bolus size and consistency affects propulsion speed suggesting that propulsion is not a simple reflex but rather a more complex process involving an adaptable neuromechanical loop.

Stimulation of synthesis and release of brain-derived neurotropic factor from intestinal smooth muscle cells by substance P and pituitary adenylate cyclase-activating peptide

BACKGROUND

Brain-derived neurotrophic factor (BDNF) is a neurotrophin present in the intestine where it participates in survival and growth of enteric neurons, augmentation of enteric circuits, and stimulation of intestinal peristalsis and propulsion. Previous studies largely focused on the role of neural and mucosal BDNF. The expression and release of BDNF from intestinal smooth muscle and the interaction with enteric neuropeptides has not been studied in gut.

METHODS

The expression and secretion of BDNF from smooth muscle cultured from the rabbit intestinal longitudinal muscle layer in response to substance P (SP) and pituitary adenylate cyclase-activating peptide (PACAP) was measured by western blot and enzyme-linked immunosorbent assay. BDNF mRNA was measured by reverse-transcription polymerase chain reaction.

KEY RESULTS

The expression of BNDF protein and mRNA was greater in smooth muscle cells (SMCs) from the longitudinal muscle than from circular muscle layer. PACAP and SP increased the expression of BDNF protein and mRNA in cultured longitudinal SMCs. PACAP and SP also stimulated the secretion of BDNF from cultured longitudinal SMCs. Chelation of intracellular calcium with BAPTA (1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) prevented SP-induced increase in BDNF mRNA and protein expression and SP-induced secretion of BDNF.

CONCLUSIONS & INFERENCES

Neuropeptides known to be present in enteric neurons innervating the longitudinal layer increase the expression of BDNF mRNA and protein in SMCs and stimulate the release of BDNF. Considering the ability of BDNF to enhance smooth muscle contraction, this autocrine loop may partially explain the characteristic hypercontractility of longitudinal muscle in inflammatory bowel disease.

Architecture and Chemical Coding of the Inner and Outer Submucous Plexus in the Colon of Piglets

In the porcine colon, the submucous plexus is divided into an inner submucous plexus (ISP) on the epithelial side and an outer submucous plexus (OSP) on the circular muscle side. Although both plexuses are probably involved in the regulation of epithelial functions, they might differ in function and neurochemical coding according to their localization. Therefore, we examined expression and co-localization of different neurotransmitters and neuronal markers in both plexuses as well as in neuronal fibres. Immunohistochemical staining was performed on wholemount preparations of ISP and OSP and on cryostat sections. Antibodies against choline acetyltransferase (ChAT), substance P (SP), somatostatin (SOM), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), neuronal nitric oxide synthase (nNOS) and the pan-neuronal markers Hu C/D and neuron specific enolase (NSE) were used. The ISP contained 1,380 ± 131 ganglia per cm² and 122 ± 12 neurons per ganglion. In contrast, the OSP showed a wider meshwork (215 ± 33 ganglia per cm²) and smaller ganglia (57 ± 3 neurons per ganglion). In the ISP, 42% of all neurons expressed ChAT. About 66% of ChAT-positive neurons co-localized SP. A small number of ISP neurons expressed SOM. Chemical coding in the OSP was more complex. Besides the ChAT/±SP subpopulation (32% of all neurons), a nNOS-immunoreactive population (31%) was detected. Most nitrergic neurons were only immunoreactive for nNOS; 10% co-localized with VIP. A small subpopulation of OSP neurons was immunoreactive for ChAT/nNOS/±VIP. All types of neurotransmitters found in the ISP or OSP were also detected in neuronal fibres within the mucosa. We suppose that the cholinergic population in the ISP is involved in the control of epithelial functions. Regarding neurochemical coding, the OSP shares some similarities with the myenteric plexus. Because of its location and neurochemical characteristics, the OSP may be involved in controlling both the mucosa and circular muscle.

Colitis-induced neuroplasticity disrupts motility in the inflamed and post-inflamed colon

Effective colonic motility involves an intricate pattern of excitatory and inhibitory neuromuscular signals that arise from the enteric neural circuitry of the colon. Recent investigations have demonstrated that inflammation leads to a variety of changes in the physiological properties of the neurons in this circuitry, including hyperexcitability of neurons at the afferent end of the peristaltic reflex, synaptic facilitation, and attenuated inhibitory neuromuscular transmission. Furthermore, links have been established between these changes and disrupted motor activity in the colon, and we now know that some of these changes persist long after recovery from inflammation. It is highly likely that inflammation-induced neuroplasticity, which is not detectable by clinical diagnostics, contributes to disrupted motility in active and quiescent inflammatory bowel disease and in functional gastrointestinal disorders.

Emerging roles of gut microbiota and the immune system in the development of the enteric nervous system

The enteric nervous system (ENS) consists of neurons and glial cells that differentiate from neural crest progenitors. During embryogenesis, development of the ENS is controlled by the interplay of neural crest cell-intrinsic factors and instructive cues from the surrounding gut mesenchyme. However, postnatal ENS development occurs in a different context, which is characterized by the presence of microbiota and an extensive immune system, suggesting an important role of these factors on enteric neural circuit formation and function. Initial reports confirm this idea while further studies in this area promise new insights into ENS physiology and pathophysiology.

Mathematical modelling of enteric neural motor patterns

1. The enteric nervous system modulates intestinal behaviours, such as motor patterns and secretion. Although much is known about different types of neurons and simple reflexes in the intestine, it remains unclear how complex behaviours are generated. 2. Mathematical modelling is an important tool for assisting the understanding of how the neurons and reflexes can be pieced together to generate intestinal behaviours. 3. Models have identified a functional role for slow excitatory post-synaptic potentials (EPSPs) by distinguishing between fast and slow EPSPs in the ascending excitation reflex. These models also discovered coordinated firing of similarly located neurons as emergent properties of feed-forward networks of interneurons in the intestine. A model of the recurrent network of intrinsic sensory neurons identified important control mechanisms to prevent uncontrolled firing due to positive feedback and that the interaction between these control mechanisms and slow EPSPs is necessary for the networks to encode ongoing sensory stimuli. This model also showed that such networks may mediate migrating motor complexes. 4. A network model of vasoactive intestinal peptide neurons in the submucosal plexus found this relatively sparse recurrent network could produce uncontrolled firing under conditions that appear to be related to cholera toxin-induced hypersecretion. 5. Abstract modelling of the intestinal fed-state motor patterns has identified how stationary contractions can arise from a polarized network. 6. These models have also helped predict and/or explained pharmacological evidence for two rhythm generators and the requirement of feedback from contractions in the circular muscle.

Molecular physiology of enteric opioid receptors

Opioid drugs have powerful antidiarrheal effects and many patients taking these drugs for chronic pain relief experience chronic constipation that can progress to opioid-induced bowel dysfunction. Three classes of opioid receptors are expressed by enteric neurons: μ-, δ-, and κ-opioid receptors (MOR, DOR, and KOR). MOR and DOR couple to inhibition of adenylate cylase and nerve terminal Ca(2+) channels and activation of K(+) channels. These effects reduce neuronal activity and neurotransmitter release. KOR couples to inhibition of Ca(2+) channels and inhibition of neurotransmitter release. In the human gastrointestinal tract, MOR, DOR, and KOR link to inhibition of acetylcholine release from enteric interneurons and purine/nitric oxide release from inhibitory motorneurons. These actions inhibit propulsive motility. MOR and DOR also link to inhibition of submucosal secretomotor neurons, reducing active Cl(-) secretion and passive water movement into the colonic lumen. These effects account for the constipation caused by opioid receptor agonists. Tolerance develops to the analgesic effects of opioid receptor agonists but not to the constipating actions. This may be due to differential β-arrestin-2-dependent opioid receptor desensitization and internalization in enteric nerves in the colon compared with the small intestine and in neuronal pain pathways. Further studies of differential opioid receptor desensitization and tolerance in subsets of enteric neurons may identify new drugs or other treatment strategies of opioid-induced bowel dysfunction.

Birthdating of myenteric neuron subtypes in the small intestine of the mouse

There are many different types of enteric neurons. Previous studies have identified the time at which some enteric neuron subtypes are born (exit the cell cycle) in the mouse, but the birthdates of some major enteric neuron subtypes are still incompletely characterized or unknown. We combined 5-ethynynl-2'-deoxyuridine (EdU) labeling with antibody markers that identify myenteric neuron subtypes to determine when neuron subtypes are born in the mouse small intestine. We found that different neurochemical classes of enteric neuron differed in their birthdates; serotonin neurons were born first with peak cell cycle exit at E11.5, followed by neurofilament-M neurons, calcitonin gene-related peptide neurons (peak cell cycle exit for both at embryonic day [E]12.5-E13.5), tyrosine hydroxylase neurons (E15.5), nitric oxide synthase 1 (NOS1) neurons (E15.5), and calretinin neurons (postnatal day [P]0). The vast majority of myenteric neurons had exited the cell cycle by P10. We did not observe any EdU+/NOS1+ myenteric neurons in the small intestine of adult mice following EdU injection at E10.5 or E11.5, which was unexpected, as previous studies have shown that NOS1 neurons are present in E11.5 mice. Studies using the proliferation marker Ki67 revealed that very few NOS1 neurons in the E11.5 and E12.5 gut were proliferating. However, Cre-lox-based genetic fate-mapping revealed a small subpopulation of myenteric neurons that appears to express NOS1 only transiently. Together, our results confirm a relationship between enteric neuron subtype and birthdate, and suggest that some enteric neurons exhibit neurochemical phenotypes during development that are different from their mature phenotype.

Neural mechanisms of peristalsis in the isolated rabbit distal colon: a neuromechanical loop hypothesis

Propulsive contractions of circular muscle are largely responsible for the movements of content along the digestive tract. Mechanical and electrophysiological recordings of isolated colonic circular muscle have demonstrated that localized distension activates ascending and descending interneuronal pathways, evoking contraction orally and relaxation anally. These polarized enteric reflex pathways can theoretically be sequentially activated by the mechanical stimulation of the advancing contents. Here, we test the hypothesis that initiation and propagation of peristaltic contractions involves a neuromechanical loop; that is an initial gut distension activates local and oral reflex contraction and anal reflex relaxation, the subsequent movement of content then acts as new mechanical stimulus triggering sequentially reflex contractions/relaxations at each point of the gut resulting in a propulsive peristaltic contraction. In fluid filled isolated rabbit distal colon, we combined spatiotemporal mapping of gut diameter and intraluminal pressure with a new analytical method, allowing us to identify when and where active (neurally-driven) contraction or relaxation occurs. Our data indicate that gut dilation is associated with propagating peristaltic contractions, and that the associated level of dilation is greater than that preceding non-propagating contractions (2.7 ± 1.4 mm vs. 1.6 ± 1.2 mm; P < 0.0001). These propagating contractions lead to the formation of boluses that are propelled by oral active neurally driven contractions. The propelled boluses also activate neurally driven anal relaxations, in a diameter dependent manner. These data support the hypothesis that neural peristalsis is the consequence of the activation of a functional loop involving mechanical dilation which activates polarized enteric circuits. These produce propulsion of the bolus which activates further anally, polarized enteric circuits by distension, thus closing the neuromechanical loop.

Properties of cholinergic and non-cholinergic submucosal neurons along the mouse colon

Submucosal neurons are vital regulators of water and electrolyte secretion and local blood flow in the gut. Due to the availability of transgenic models for enteric neuropathies, the mouse has emerged as the research model of choice, but much is still unknown about the murine submucosal plexus. The progeny of choline acetyltransferase (ChAT)-Cre × ROSA26(YFP) reporter mice, ChAT-Cre;R26R-yellow fluorescent protein (YFP) mice, express YFP in every neuron that has ever expressed ChAT. With the aid of the robust YFP staining in these mice, we correlated the neurochemistry, morphology and electrophysiology of submucosal neurons in distal colon. We also examined whether there are differences in neurochemistry along the colon and in neurally mediated vectorial ion transport between the proximal and distal colon. All YFP(+) submucosal neurons also contained ChAT. Two main neurochemical but not electrophysiological groups of neurons were identified: cholinergic (containing ChAT) or non-cholinergic. The vast majority of neurons in the middle and distal colon were non-cholinergic but contained vasoactive intestinal peptide. In the distal colon, non-cholinergic neurons had one or two axons, whereas the cholinergic neurons examined had only one axon. All submucosal neurons exhibited S-type electrophysiology, shown by the lack of long after-hyperpolarizing potentials following their action potentials and fast excitatory postsynaptic potentials (EPSPs). Fast EPSPs were predominantly nicotinic, and somatic action potentials were mediated by tetrodotoxin-resistant voltage-gated channels. The size of submucosal ganglia decreased but the proportion of cholinergic neurons increased distally along the colon. The distal colon had a significantly larger nicotinic ion transport response than the proximal colon. This work shows that the properties of murine submucosal neurons and their control of epithelial ion transport differ between colonic regions. There are several key differences between the murine submucous plexus and that of other animals, including a lack of conventional intrinsic sensory neurons, which suggests there is an incomplete neuronal circuitry within the murine submucous plexus.

Effects of inflammation on the innervation of the colon

Inflammatory bowel diseases (IBD) such as ulcerative colitis and Crohn's disease lead to altered gastrointestinal (GI) function as a consequence of the effects of inflammation on the tissues that comprise the GI tract. Among these tissues are several types of neurons that detect the state of the GI tract, transmit pain, and regulate functions such as motility, secretion, and blood flow. This review article describes the structure and function of the enteric nervous system, which is embedded within the gut wall, the sympathetic motor innervation of the colon and the extrinsic afferent innervation of the colon, and considers the evidence that colitis alters these important sensory and motor systems. These alterations may contribute to the pain and altered bowel habits that accompany IBD.

Substance P as a neuronal factor in the enteric nervous system of the porcine descending colon in physiological conditions and during selected pathogenic processes

The present investigation pertains to changes in substance P-like immunoreactive (SP-LI) nerve structures of the enteric nervous system (ENS) in the porcine descending colon, caused by chemically-induced inflammation and nerve injury (axotomy). The distribution pattern of SP-LI structures was studied using the double immunofluorescence technique in the myenteric (MP), outer submucous (OSP) and inner submucous (ISP) plexuses, as well as in the circular muscle and mucosal layers. Under physiological conditions, SP-LI neurons have been shown to constitute 4.13 ± 0.24%, 3.36 ± 0.26%, and 7.92 ± 0.16% in the MP, OSP, and ISP, respectively. Changes in SP-immunoreactivity depended on the pathological factor studied. The numbers of the SP-LI perikarya amounted to 7.89 ± 0.34, 5.56 ± 0.30, and 19.96 ± 0.57 in chemically-induced colitis, and 4.28 ± 0.13%, 7.18 ± 20%, and 11.62 ± 0.48% after axotomy in MP, OSP, and ISP, respectively. The both studied processes generally resulted in an increase in the number of SP-LI nerve fibers in the circular muscle and mucosal layers. The obtained results suggest that SP-LI nerve structures of the ENS may participate in various pathological processes in the porcine descending colon and exact functions of SP probably depend on the type of the pathological factor.

Neurochemistry of myenteric plexus neurons of bank vole (Myodes glareolus) ileum

The neurochemistry of enteric neurons differs among species of small laboratory rodents (guinea-pig, mouse, rat). In this study we characterized the phenotype of ileal myenteric plexus (MP) neuronal cells and fibers of the bank vole (Myodes glareolus), a common rodent living in Europe and in Northern Asia which is also employed in prion experimental transmission studies. Six neuronal markers were tested: choline acetyltransferase (ChAT), neuronal nitric oxide synthase (nNOS), calbindin (CALB), calcitonin gene-related peptide (CGRP) and substance P (SP), along with HuC/D as a pan-neuronal marker. Neurons expressing ChAT- and nNOS-immunoreactivity (IR) were 36 ± 12% and 24 ± 5%, respectively. Those expressing CGRP-, SP- and CALB-IR were 3 ± 3%, 21 ± 5% and 6 ± 2%, respectively. Therefore, bank vole MPs differ consistently from murine MPs in neurons expressing CGRP-, SP- and CALB-IR. These data may contribute to define the prion susceptibility of neuron cell populations residing within ileal MPs from bank voles, along with their morpho-functional alterations following oral experimental prion challenge.

Expression of neuropeptides and anoctamin 1 in the embryonic and adult zebrafish intestine, revealing neuronal subpopulations and ICC-like cells

This immunohistochemical study in zebrafish aims to extend the neurochemical characterization of enteric neuronal subpopulations and to validate a marker for identification of interstitial cells of Cajal (ICC). The expression of neuropeptides and anoctamin 1 (Ano1), a selective ICC marker in mammals, was analyzed in both embryonic and adult intestine. Neuropeptides were present from 3 days postfertilization (dpf). At 3 dpf, galanin-positive nerve fibers were found in the proximal intestine, while calcitonin gene-related peptide (CGRP)- and substance P-expressing fibers appeared in the distal intestine. At 5 dpf, immunoreactive fibers were present along the entire intestinal length, indicating a well-developed peptidergic innervation at the onset of feeding. In the adult intestine, vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating peptide (PACAP), galanin, CGRP and substance P were detected in nerve fibers. Colchicine pretreatment enhanced only VIP and PACAP immunoreactivity. VIP and PACAP were coexpressed in enteric neurons. Colocalization stainings revealed three neuronal subpopulations expressing VIP and PACAP: a nitrergic noncholinergic subpopulation, a serotonergic subpopulation and a subpopulation expressing no other markers. Ano1-immunostaining revealed a 3-dimensional network in the adult intestine containing multipolar cells at the myenteric plexus and bipolar cells interspersed between circular smooth muscle cells. Ano1 immunoreactivity first appeared at 3 dpf, indicative of the onset of proliferation of ICC-like cells. It is shown that the Ano1 antiserum is a selective marker of ICC-like cells in the zebrafish intestine. Finally, it is hypothesized that ICC-like cells mediate the spontaneous regular activity of the embryonic intestine.

Functional organization of autonomic neural pathways

There is now abundant functional and anatomical evidence that autonomic motor pathways represent a highly organized output of the central nervous system. Simplistic notions of antagonistic all-or-none activation of sympathetic or parasympathetic pathways are clearly wrong. Sympathetic or parasympathetic pathways to specific target tissues generally can be activated tonically or phasically, depending on current physiological requirements. For example, at rest, many sympathetic pathways are tonically active, such as those limiting blood flow to the skin, inhibiting gastrointestinal tract motility and secretion, or allowing continence in the urinary bladder. Phasic parasympathetic activity can be seen in lacrimation, salivation or urination. Activity in autonomic motor pathways can be modulated by diverse sensory inputs, including the visual, auditory and vestibular systems, in addition to various functional populations of visceral afferents. Identifying the central pathways responsible for coordinated autonomic activity has made considerable progress, but much more needs to be done.

Oxidative stress disrupts purinergic neuromuscular transmission in the inflamed colon

Colitis, induced by trinitrobenzene sulfonic acid (TNBS) in guinea pig, leads to decreased purinergic neuromuscular transmission resulting in a reduction in inhibitory junction potentials (IJPs) in colonic circular muscle. We explored possible mechanisms responsible for this inflammation-induced neurotransmitter plasticity. Previous studies have suggested that the deficit in inflamed tissue involves decreased ATP release. We therefore hypothesized that decreased purinergic transmission results from inflammation-induced free radical damage to mitochondria, leading to decreased purine synthesis and release. Stimulus-induced release of purines was measured using high-performance liquid chromatography, and quantities of all purines measured were significantly reduced in the inflamed colons as compared to controls. To test whether decreased mitochondrial function affects the IJP, colonic muscularis preparations were treated with the mitochondrial ATP synthase inhibitors oligomycin or dicyclohexylcarbodiimide, which resulted in a significant reduction of IJP amplitude. Induction of oxidative stress in vitro, by addition of H2O2 to the preparation, also significantly reduced IJP amplitude. Purinergic neuromuscular transmission was significantly restored in TNBS-inflamed guinea pigs, and in dextran sodium sulfate-inflamed mice, treated with a free radical scavenger. Furthermore, propulsive motility in the distal colons of guinea pigs with TNBS colitis was improved by in vivo treatment with the free radical scavenger. We conclude that oxidative stress contributes to the reduction in purinergic neuromuscular transmission measured in animal models of colitis, and that these changes can be prevented by treatment with a free radical scavenger, resulting in improved motility.

Genetic fate-mapping of tyrosine hydroxylase-expressing cells in the enteric nervous system

BACKGROUND

During development of the enteric nervous system, a subpopulation of enteric neuron precursors transiently expresses catecholaminergic properties. The progeny of these transiently catecholaminergic (TC) cells have not been fully characterized.

METHODS

We combined in vivo Cre-lox-based genetic fate-mapping with phenotypic analysis to fate-map enteric neuron subtypes arising from tyrosine hydroxylase (TH)-expressing cells.

KEY RESULTS

Less than 3% of the total (Hu(+) ) neurons in the myenteric plexus of the small intestine of adult mice are generated from transiently TH-expressing cells. Around 50% of the neurons generated from transiently TH-expressing cells are calbindin neurons, but their progeny also include calretinin, neurofilament-M, and serotonin neurons. However, only 30% of the serotonin neurons and small subpopulations (<10%) of the calbindin, calretinin, and neurofilament-M neurons are generated from TH-expressing cells; only 0.2% of nitric oxide synthase neurons arise from TH-expressing cells.

CONCLUSIONS & INFERENCES

Transiently, catecholaminergic cells give rise to subpopulations of multiple enteric neuron subtypes, but the majority of each of the neuron subtypes arises from non-TC cells.

Planar cell polarity genes control the connectivity of enteric neurons

A highly complex network of intrinsic enteric neurons is required for the digestive and homeostatic functions of the gut. Nevertheless, the genetic and molecular mechanisms that regulate their assembly into functional neuronal circuits are currently unknown. Here we report that the planar cell polarity (PCP) genes Celsr3 and Fzd3 are required during murine embryogenesis to specifically control the guidance and growth of enteric neuronal projections relative to the longitudinal and radial gut axes. Ablation of these genes disrupts the normal organization of nascent neuronal projections, leading to subtle changes of axonal tract configuration in the mature enteric nervous system (ENS), but profound abnormalities in gastrointestinal motility. Our data argue that PCP-dependent modules of connectivity established at early stages of enteric neurogenesis control gastrointestinal function in adult animals and provide the first evidence that developmental deficits in ENS wiring may contribute to the pathogenesis of idiopathic bowel disorders.

Differential release of β-NAD(+) and ATP upon activation of enteric motor neurons in primate and murine colons

BACKGROUND

The purinergic component of enteric inhibitory neurotransmission is important for normal motility in the gastrointestinal (GI) tract. Controversies exist about the purine(s) responsible for inhibitory responses in GI muscles: ATP has been assumed to be the purinergic neurotransmitter released from enteric inhibitory motor neurons; however, recent studies demonstrate that β-nicotinamide adenine dinucleotide (β-NAD(+)) and ADP-ribose mimic the inhibitory neurotransmitter better than ATP in primate and murine colons. The study was designed to clarify the sources of purines in colons of Cynomolgus monkeys and C57BL/6 mice.

METHODS

High-performance liquid chromatography with fluorescence detection was used to analyze purines released by stimulation of nicotinic acetylcholine receptors (nAChR) and serotonergic 5-HT(3) receptors (5-HT(3)R), known to be present on cell bodies and dendrites of neurons within the myenteric plexus.

KEY RESULTS

Nicotinic acetylcholine receptor or 5-HT(3)R agonists increased overflow of ATP and β-NAD(+) from tunica muscularis of monkey and murine colon. The agonists did not release purines from circular muscles of monkey colon lacking myenteric ganglia. Agonist-evoked overflow of β-NAD(+), but not ATP, was inhibited by tetrodotoxin (0.5 μmol L(-1)) or ω-conotoxin GVIA (50 nmol L(-1)), suggesting that β-NAD(+) release requires nerve action potentials and junctional mechanisms known to be critical for neurotransmission. ATP was likely released from nerve cell bodies in myenteric ganglia and not from nerve terminals of motor neurons.

CONCLUSIONS & INFERENCES

These results support the conclusion that ATP is not a motor neurotransmitter in the colon and are consistent with the hypothesis that β-NAD(+), or its metabolites, serve as the purinergic inhibitory neurotransmitter.

Transplanted progenitors generate functional enteric neurons in the postnatal colon

Cell therapy has the potential to treat gastrointestinal motility disorders caused by diseases of the enteric nervous system. Many studies have demonstrated that various stem/progenitor cells can give rise to functional neurons in the embryonic gut; however, it is not yet known whether transplanted neural progenitor cells can migrate, proliferate, and generate functional neurons in the postnatal bowel in vivo. We transplanted neurospheres generated from fetal and postnatal intestinal neural crest-derived cells into the colon of postnatal mice. The neurosphere-derived cells migrated, proliferated, and generated neurons and glial cells that formed ganglion-like clusters within the recipient colon. Graft-derived neurons exhibited morphological, neurochemical, and electrophysiological characteristics similar to those of enteric neurons; they received synaptic inputs; and their neurites projected to muscle layers and the enteric ganglia of the recipient mice. These findings show that transplanted enteric neural progenitor cells can generate functional enteric neurons in the postnatal bowel and advances the notion that cell therapy is a promising strategy for enteric neuropathies.

Substance P- and choline acetyltransferase immunoreactivities in somatostatin-containing, human submucosal neurons

The submucous layers of human small and large intestines contain at least two separate neuron populations. Besides morphological features, they differ in their immunoreactivities for calretinin (CALR) and somatostatin (SOM), respectively. In this study, submucosal wholemounts of 23 patients or body donors (including all segments of small intestine and colon) were immunohistochemically quadruple stained for CALR and SOM as well as for substance P (SP) and choline acetyltransferase (ChAT). We found that all SOM-positive neurons co-stained for ChAT and the majority for SP [between 50% in the small intestinal external submucosal plexus (ESP) and 75% in the colonic ESP]. In contrast, a majority of CALR-neurons contained ChAT (between 77% in the small intestinal ESP and 92% in the large intestinal ESP) whereas less than 4% of CALR-neurons were co-immunoreactive for SP. Another set of wholemounts was co-stained for peripherin, a marker enabling morphological analysis. Where identifiable, both SOM alone- and SOM/SP-neurons displayed a uniaxonal (supposed pseudouniaxonal) morphology. We suggest that the chemical code of SOM-immunoreactive, human submucosal neurons may be "ChAT+/SOM+/SP±". In additional sections double stained for SOM and SP, we regularly found double-labelled nerve fibres only in the mucosa. In contrast, around submucosal arteries mostly SOM alone- fibres were found and the muscularis propria contained numerous SP-alone fibres. We conclude that the main target of submucosal SOM(/SP)-neurons may be the mucosa. Due to their morpho-chemical similarity to human myenteric type II neurons, we further suggest that one function of human submucosal SOM-neurons may be a primary afferent one.

The emergence of neural activity and its role in the development of the enteric nervous system

The enteric nervous system (ENS) is a vital part of the autonomic nervous system that regulates many gastrointestinal functions, including motility and secretion. All neurons and glia of the ENS arise from neural crest-derived cells that migrate into the gastrointestinal tract during embryonic development. It has been known for many years that a subpopulation of the enteric neural crest-derived cells expresses pan-neuronal markers at early stages of ENS development. Recent studies have demonstrated that some enteric neurons exhibit electrical activity from as early as E11.5 in the mouse, with further maturation of activity during embryonic and postnatal development. This article discusses the maturation of electrophysiological and morphological properties of enteric neurons, the formation of synapses and synaptic activity, and the influence of neural activity on ENS development.

The NOP receptor involvement in both withdrawal- and CCk-8-induced contracture responses of guinea pig isolated ileum after acute activation of κ-opioid receptor

In isolated guinea-pig ileum (GPI), the κ-opioid acute withdrawal response is under the control of several neuronal signaling systems, including the μ-opioid, the A(1)-adenosine and the CB(1) receptors, which are involved in the inhibitory control of the κ-withdrawal response. After κ-opioid system stimulation, indirect activation of μ-opioid, A(1)-adenosine and CB(1) systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the NOP system is also involved in the regulation of the acute κ-withdrawal response. Interestingly, we found that in GPI preparation, the NOP system is not indirectly activated by the κ-opioid receptor stimulation, but instead this system is able by itself to directly regulate the acute κ-withdrawal response. Specifically, our results clearly highlight first the existence of an endogenous tone of the NOP system in GPI, and second that it behaves as a functional anti-opioid system. We also found that, the NOP receptor system is involved in the regulation of the CCk-8-induced contracture intensity, only when in the presence of the κ-opioid receptor stimulation. This effect seems to be regulated by an activation threshold mechanism. In conclusion, the NOP system could act as neuromodulatory system, whose action is strictly related to the modulation of both excitatory and inhibitory neurotransmitters released in GPI enteric nervous system.

Building a brain in the gut: development of the enteric nervous system

The enteric nervous system (ENS), the intrinsic innervation of the gastrointestinal tract, is an essential component of the gut neuromusculature and controls many aspects of gut function, including coordinated muscular peristalsis. The ENS is entirely derived from neural crest cells (NCC) which undergo a number of key processes, including extensive migration into and along the gut, proliferation, and differentiation into enteric neurons and glia, during embryogenesis and fetal life. These mechanisms are under the molecular control of numerous signaling pathways, transcription factors, neurotrophic factors and extracellular matrix components. Failure in these processes and consequent abnormal ENS development can result in so-called enteric neuropathies, arguably the best characterized of which is the congenital disorder Hirschsprung disease (HSCR), or aganglionic megacolon. This review focuses on the molecular and genetic factors regulating ENS development from NCC, the clinical genetics of HSCR and its associated syndromes, and recent advances aimed at improving our understanding and treatment of enteric neuropathies.

Neurochemical characterization of zinc transporter 3-like immunoreactive (ZnT3(+)) neurons in the intramural ganglia of the porcine duodenum

The SLC30 family of divalent cation transporters is thought to be involved in the transport of zinc in a variety of cellular pathways. Zinc transporter 3 (ZnT3) is involved in the transport of zinc into synaptic vesicles or intracellular organelles. As the presence of ZnT3 immunoreactive neurons has recently been reported in both the central and peripheral nervous systems of the rat, the present study was aimed at disclosing the presence of a zinc-enriched neuron enteric population in the porcine duodenum to establish a preliminary insight into their neurochemical coding. Double- and triple-immunofluorescence labeling of the porcine duodenum for ZnT3 with the pan-neuronal marker (PGP 9.5), substance P, somatostatin, vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS), leu-enkephalin, vesicular acetylcholine transporter (VAChT), neuropeptide Y, galanin (GAL), and calcitonin gene-related peptide were performed. Immunohistochemistry revealed that approximately 35, 43, and 48 % of all PGP9.5-postive neurons in the myenteric (MP), outer submucous (OSP), and inner submucous (ISP) plexuses, respectively, of the porcine duodenum were simultaneously ZnT3⁺. In the present study, ZnT3⁺ neurons coexpressed a broad spectrum of active substances, but co-localization patterns unique to the plexus were studied. In the ISP, all ZnT3⁺ neurons were VAChT positive, and the largest populations among these cells formed ZnT3⁺/VAChT⁺/GAL⁺ and ZnT3⁺/VAChT⁺/VIP⁺ cells. In the OSP and MP, the numbers of ZnT3⁺/VAChT⁺ neurons were two times smaller, and substantial subpopulations of ZnT3⁺ neurons in both these plexuses formed ZnT3⁺/NOS⁺ cells. The large population of ZnT3⁺ neurons in the porcine duodenum and a broad spectrum of active substances which co-localize with this peptide suggest that ZnT3 takes part in the regulation of various processes in the gut both in normal physiology and during pathological processes.

Effects of aging on cholinergic neuromuscular transmission in isolated small intestine of ad libitum fed and calorically-restricted rats

BACKGROUND

Age-associated losses of enteric neurons have been described. In rat ileum, myenteric neurons lost during aging have been reported to be predominantly cholinergic, and caloric restriction (CR) has been shown to protect against these losses. Cholinergic myenteric neurons include excitatory motor neurons, so the aim of this work was to determine whether neuronal loss in ad libitum (AL)-fed animals is reflected in dysfunctional cholinergic neuromuscular transmission, and if CR reduces any such dysfunction.

METHODS

Effects of electrical field stimulation (EFS) and applied acetylcholine (ACh) were examined in the longitudinal muscle of isolated ileal segments from 6-month-old rats and from 13- and 24-month-old rats fed either AL or CR diets.

KEY RESULTS

Contractile responses to EFS were abolished by atropine and potentiated by the acetylcholinesterase inhibitor, eserine. Frequency-response relationships were not significantly different amongst the three age-groups. Sensitivity to applied ACh, however, was three-fold lower in the oldest animals (P < 0.05). Eserine potentiated responses to ACh; there were no statistically significant differences amongst the sensitivities to ACh in its presence. No significant differences between AL- and CR-fed animals were measured, although variability was less in CR-fed than in AL-fed groups.

CONCLUSIONS & INFERENCES

The cholinergic system supplying the rat ileum longitudinal muscle did not appear to be impaired in old age. Decreased sensitivity to applied ACh in old tissues may have been due to increased acetylcholinesterase activity. Caloric restriction had no significant effect on responses to EFS or applied ACh. The implications of these results are discussed.

Differing effects of NT-3 and GDNF on dissociated enteric ganglion cells exposed to hydrogen peroxide in vitro

Oxidative stress is widely recognized to contribute to neuronal death during various pathological conditions and ageing. In the enteric nervous system (ENS), reactive oxygen species have been implicated in the mechanism of age-associated neuronal loss. The neurotrophic factors, neurotrophin 3 (NT-3) and glial cell line-derived neurotrophic factor (GDNF), are important in the development of enteric neurons and continue to be expressed in the gut throughout life. It has therefore been suggested that they may have a neuroprotective role in the ENS. We investigated the potential of NT-3 and GDNF to prevent the death of enteric ganglion cells in dissociated cell culture after exposure to hydrogen peroxide (H(2)O(2)). H(2)O(2) treatment resulted in a dose-dependent death of enteric neurons and glial cells, as demonstrated by MTS assay, bis-benzimide and propidium iodide staining and immunolabelling. Cultures treated with NT-3 prior to exposure showed reduced cell death compared to untreated control or GDNF-treated cultures. GDNF treatment did not affect neuronal survival in H(2)O(2)-treated cultures. These results suggest that NT-3 is able to enhance the survival of enteric ganglion cells exposed to oxidative stress.

Preferential entry of botulinum neurotoxin A Hc domain through intestinal crypt cells and targeting to cholinergic neurons of the mouse intestine

Botulism, characterized by flaccid paralysis, commonly results from botulinum neurotoxin (BoNT) absorption across the epithelial barrier from the digestive tract and then dissemination through the blood circulation to target autonomic and motor nerve terminals. The trafficking pathway of BoNT/A passage through the intestinal barrier is not yet fully understood. We report that intralumenal administration of purified BoNT/A into mouse ileum segment impaired spontaneous muscle contractions and abolished the smooth muscle contractions evoked by electric field stimulation. Entry of BoNT/A into the mouse upper small intestine was monitored with fluorescent HcA (half C-terminal domain of heavy chain) which interacts with cell surface receptor(s). We show that HcA preferentially recognizes a subset of neuroendocrine intestinal crypt cells, which probably represent the entry site of the toxin through the intestinal barrier, then targets specific neurons in the submucosa and later (90–120 min) in the musculosa. HcA mainly binds to certain cholinergic neurons of both submucosal and myenteric plexuses, but also recognizes, although to a lower extent, other neuronal cells including glutamatergic and serotoninergic neurons in the submucosa. Intestinal cholinergic neuron targeting by HcA could account for the inhibition of intestinal peristaltism and secretion observed in botulism, but the consequences of the targeting to non-cholinergic neurons remains to be determined.

Botulism is a severe and often fatal disease in man and animals characterized by flaccid paralysis. Clostridium botulinum produces a potent neurotoxin (botulinum neurotoxin) responsible for all the symptoms of botulism. Botulism is most often acquired by ingesting preformed botulinum neurotoxin in contaminated food or after intestinal colonization by C. botulinum under certain circumstances, such as in infant botulism, and toxin production in the intestine. The first step of the disease consists in the passage of the botulinum neurotoxin through the intestinal barrier, which is still poorly understood. We investigated the trafficking of the botulinum neurotoxin in a mouse intestinal loop model, using fluorescent HcA (half C-terminal domain of the heavy chain). We observed that HcA preferentially recognizes neuroendocrine intestinal crypt cells, which likely represent the entry site of the toxin through the intestinal barrier, then targets specific neurons, mainly cholinergic neurons, in the submucosa, and later (90–120 min) in the musculosa leading to local paralytic effects such as inhibition of intestinal peristaltism. These results represent an important advance in the understanding of the initial steps of botulism intoxication and can be the basis for the development of new specific countermeasures against botulism.

The enteric nervous system and neurogastroenterology

Neurogastroenterology is defined as neurology of the gastrointestinal tract, liver, gallbladder and pancreas and encompasses control of digestion through the enteric nervous system (ENS), the central nervous system (CNS) and integrative centers in sympathetic ganglia. This Review provides a broad overview of the field of neurogastroenterology, with a focus on the roles of the ENS in the control of the musculature of the gastrointestinal tract and transmucosal fluid movement. Digestion is controlled through the integration of multiple signals from the ENS and CNS; neural signals also pass between distinct gut regions to coordinate digestive activity. Moreover, neural and endocrine control of digestion is closely coordinated. Interestingly, the extent to which the ENS or CNS controls digestion differs considerably along the digestive tract. The importance of the ENS is emphasized by the life-threatening effects of certain ENS neuropathies, including Hirschsprung disease and Chagas disease. Other ENS disorders, such as esophageal achalasia and gastroparesis, cause varying degrees of dysfunction. The neurons in enteric reflex pathways use a wide range of chemical messengers that signal through an even wider range of receptors. These receptors provide many actual and potential targets for modifying digestive function.

Glucagon-like peptide-1 modulates neurally evoked mucosal chloride secretion in guinea pig small intestine in vitro

Glucagon-like peptide-1 (GLP-1) acts at the G protein-coupled receptor, GLP-1R, to stimulate secretion of insulin and to inhibit secretion of glucagon and gastric acid. Involvement in mucosal secretory physiology has received negligible attention. We aimed to study involvement of GLP-1 in mucosal chloride secretion in the small intestine. Ussing chamber methods, in concert with transmural electrical field stimulation (EFS), were used to study actions on neurogenic chloride secretion. ELISA was used to study GLP-1R effects on neural release of acetylcholine (ACh). Intramural localization of GLP-1R was assessed with immunohistochemistry. Application of GLP-1 to serosal or mucosal sides of flat-sheet preparations in Ussing chambers did not change baseline short-circuit current (I(sc)), which served as a marker for chloride secretion. Transmural EFS evoked neurally mediated biphasic increases in I(sc) that had an initial spike-like rising phase followed by a sustained plateau-like phase. Blockade of the EFS-evoked responses by tetrodotoxin indicated that the responses were neurally mediated. Application of GLP-1 reduced the EFS-evoked biphasic responses in a concentration-dependent manner. The GLP-1 receptor antagonist exendin-(9-39) suppressed this action of GLP-1. The GLP-1 inhibitory action on EFS-evoked responses persisted in the presence of nicotinic or vasoactive intestinal peptide receptor antagonists but not in the presence of a muscarinic receptor antagonist. GLP-1 significantly reduced EFS-evoked ACh release. In the submucosal plexus, GLP-1R immunoreactivity (IR) was expressed by choline acetyltransferase-IR neurons, neuropeptide Y-IR neurons, somatostatin-IR neurons, and vasoactive intestinal peptide-IR neurons. Our results suggest that GLP-1R is expressed in guinea pig submucosal neurons and that its activation leads to a decrease in neurally evoked chloride secretion by suppressing release of ACh at neuroepithelial junctions in the enteric neural networks that control secretomotor functions.

Biphasic regulation of the acute μ-withdrawal and CCk-8 contracture responses by the ORL-1 system in guinea pig ileum

The cloning of the opioid-receptor-like receptor (ORL-1) and the identification of the orphaninFQ/nociceptin (OFQ/N) as its endogenous agonist has revealed a new G-protein-coupled receptor signalling system. The structural and functional homology of ORL-1 to the opioid receptor systems has posed a number of challenges in the understanding the often competing physiological responses elicited by these G-protein-coupled receptors. We had previously shown that in guinea pig ileum (GPI), the acute μ-withdrawal response is under the inhibitory control of several systems. Specifically, we found that the exposure to a μ-opioid receptor agonist activates indirectly the κ-opioid, the A(1)-adenosine and the cannabinoid CB(1) systems, that in turn inhibit the withdrawal response. The indirect activation of these systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the ORL-1 system is also involved in the regulation of the acute μ-withdrawal response. Interestingly, we found that in GPI preparation, the ORL-1 system is not indirectly activated by the μ-opioid receptor stimulation, but instead the system is able by itself to directly regulate the acute μ-withdrawal response. Moreover, we have demonstrated that the ORL-1 system behaves both as anti-opioid or opioid-like system based on the level of activation. The same behaviour has also been observed in presence of CCk-8. Furthermore, in GPI, the existence of an endogenous tone of the ORL-1 system has been demonstrated. We concluded that the ORL-1 system acts as a neuromodulatory system, whose action is strictly related to the modulation of excitatory neurotrasmitters released in GPI enteric nervous system.

COMPUTER SIMULATION OF COTRANSMISSION BY EXCITATORY AMINO ACIDS AND ACETYLCHOLINE IN THE ENTERIC NERVOUS SYSTEM

The role of cotransmission by α-amino-3-hydroxy-5-methyl-4-isoxalose propionic acid (AMPA), L-aspartate, N-methyl-D-aspartate (NMDA), and acetylcholine (ACh) as well as the coexpression of AMPA, NMDA, and nicotinic ACh (nACh) receptors on the electrophysiological activity of the primary sensory (AH) and motor (S) neurons of the enteric nervous system are numerically assessed. Results of computer simulations showed that AMPA and L-Asp alone can induce fast action potentials of short duration on AH and S neurons. Costimulation of nACh and AMPA receptors on the soma of the S neuron resulted in periodic spiking activity. A characteristic biphasic response was recorded from the AH neuron after coactivation of AMPA and NMDA receptors. Glutamate alone acting on NMDA receptors caused prolonged depolarization of the AH neuron and failed to depolarize the S neuron. Cojoint stimulation of the AMPA or nACh receptors was required to produce the effect of glutamate. The overall electrical response of neurons to the activation of NMDA receptors was long-term depolarization. Acetylcholine, AMPA, and glutamate acting alone or cojointly enhanced phasic contraction of the longitudinal smooth muscle. Treatment of neurons with AMPA, NMDA, and nACh receptor antagonists revealed intricate properties of the AH and S neurons. Application of MK-801, D-AP5, and CPP reduced the excitability of the AH neuron and totally abolished electrical activity in the S neuron. The information gained into the cotransmission by excitatory amino acids and acetylcholine in the enteric nervous system may be beneficial in the development of novel effective therapeutics to treat diseases associated with altered visceral nociception, i.e. irritable bowel syndrome.

Nonruminant Nutrition Symposium: Neurogastroenterology and food allergies

Neurogastroenterology is a subspecialty encompassing relations of the nervous system to the gastrointestinal tract. The central concept is emergence of whole organ behavior from coordinated activity of the musculature, mucosal epithelium, and blood vasculature. Behavior of each effector is determined by the enteric nervous system (ENS). The ENS is a minibrain positioned close to the effectors it controls. The ENS neurophysiology is in the framework of neurogastroenterology. The digestive tract is recognized as the largest lymphoid organ in the body with a unique complement of mast cells. In its position at the "dirtiest" of interfaces between the body and outside world, the mucosal immune system encounters food antigens, bacteria, parasites, viruses, and toxins. Epithelial barriers are insufficient to exclude fully the antigenic load, thereby allowing chronic challenges to the immune system. Observations in antigen-sensitized animals document direct communication between the mucosal immune system and ENS. Communication is functional and results in adaptive responses to circumstances within the lumen that are threatening to the functional integrity of the whole animal. Communication is paracrine and incorporates specialized sensing functions of mast cells for specific antigens together with the capacity of the ENS for intelligent interpretation of the signals. Immuno-neural integration progresses sequentially, beginning with immune detection, followed by signal transfer to the ENS, followed by neural interpretation and then selection of a neural program with coordinated mucosal secretion and a propulsive motor event that quickly clears the threat from the intestinal lumen. Operation of the defense program evokes symptoms of cramping abdominal pain, fecal urgency, and acute watery diarrhea. Investigative approaches to immuno-ENS interactions merge the disciplines of mucosal immunology and ENS neurophysiology into the realm of neurogastroenterology.

Antinociceptive effect of VSL#3 on visceral hypersensitivity in a rat model of irritable bowel syndrome: a possible action through nitric oxide pathway and enhance barrier function

Irritable bowel syndrome (IBS) is a functional bowel disorder characterized by visceral hypersensitivity and altered bowel function. There are increasing evidences suggested that VSL#3 probiotics therapy has been recognized as an effective method to relieve IBS-induced symptoms. The aim of this study was to examine the effects of VSL#3 probiotics on visceral hypersensitivity (VH), nitric oxide (NO), fecal character, colonic epithelium permeability, and tight junction protein expression. IBS model was induced by intracolonic instillation of 4% acetic acid and restraint stress in rats. After subsidence of inflammation on the seventh experimental day, the rats were subjected to rectal distension, and then the abdominal withdrawal reflex and the number of fecal output were measured, respectively. Also, colonic permeability to Evans blue was measured in vivo, and tight junction protein expression was studied by immunohistochemistry and immunoblotting method. Rats had been pretreated with VSL#3 or aminoguanidine (NOS inhibitor) or VSL#3+ aminoguanidine before measurements. The rats at placebo group showed hypersensitive response to rectal distension (P < 0.05) and defecated more stools than control rats (P < 0.05), whereas VSL#3 treatment significantly attenuated VH and effectively reduced defecation. Aminoguanidine reduced the protective effects of VSL#3 on VH. A pronounced increase in epithelial permeability and decreased expression of tight junction proteins (occludin, ZO-1) in placebo group were prevented by VSL#3, but not aminoguanidine. VSL#3 treatment reduce the hypersensitivity, defecation, colonic permeability and increase the expression of tight junction proteins (occludin, ZO-1). As the part of this effect was lowered by NOS inhibitor, NO might play a role in the protective effect of VSL#3 to some extent.

Neostigmine-induced contraction and nitric oxide-induced relaxation of isolated ileum from STZ diabetic guinea pigs

Both delayed gastrointestinal transit and autonomic neuropathy have been documented in patients with diabetes mellitus. The mechanism of neostigmine, an agent that mimics release of acetylcholine from autonomic neurons by prokinetic agents, to contract smooth muscle, despite dysfunctional enteric neural pathways, was determined using isolated ilea from STZ-treated and control guinea pigs. Both bethanechol- and neostigmine-induced contractions were stronger in diabetic ileum. Bethanechol-induced contractions of control but not diabetic ileum were increased by low dose scopolamine suggesting reduced activation of presynaptic muscarinic autoreceptors in diabetic ileum. The muscarinic receptor antagonist 4-DAMP strongly, but the nicotinic receptor antagonist hexamethonium only weakly, reduced neostigmine-induced contractions of control and diabetic ilea. The amount of acetylcholine, inferred from tissue choline content, was increased in diabetic ileum. Nicotinic neural and noncholinergic postjunctional smooth muscle receptors contributed more strongly to neostigmine-induced contractions in diabetic than control ileum. Relaxation of diabetic ileum by exogenous nitric oxide generated from sodium nitroprusside was comparable to control ileum, but smooth muscle relaxation by l-arginine using neuronal nitric oxide synthase to generate nitric oxide was weaker in diabetic ileum with evidence for a role for inducible nitric oxide synthase. Despite autonomic neuropathy, neostigmine strongly contracted ileum from diabetic animals but by a different mechanism including stronger activation of postjunctional muscarinic receptors, greater synaptic acetylcholine, stronger activation of noncholinergic excitatory pathways, and weaker activation of inhibitory pathways. A selective medication targeting a specific neural pathway may more effectively treat disordered gastrointestinal transit in patients with diabetes mellitus.

Neural regulation of intestinal nutrient absorption

The nervous system and the gastrointestinal (GI) tract share several common features including reciprocal interconnections and several neurotransmitters and peptides known as gut peptides, neuropeptides or hormones. The processes of digestion, secretion of digestive enzymes and then absorption are regulated by the neuro-endocrine system. Luminal glucose enhances its own absorption through a neuronal reflex that involves capsaicin sensitive primary afferent (CSPA) fibres. Absorbed glucose stimulates insulin release that activates hepatoenteric neural pathways leading to an increase in the expression of glucose transporters. Adrenergic innervation increases glucose absorption through α1 and β receptors and decreases absorption through activation of α2 receptors. The vagus nerve plays an important role in the regulation of diurnal variation in transporter expression and in anticipation to food intake. Vagal CSPAs exert tonic inhibitory effects on amino acid absorption. It also plays an important role in the mediation of the inhibitory effect of intestinal amino acids on their own absorption at the level of proximal or distal segment. However, chronic extrinsic denervation leads to a decrease in intestinal amino acid absorption. Conversely, adrenergic agonists as well as activation of CSPA fibres enhance peptides uptake through the peptide transporter PEPT1. Finally, intestinal innervation plays a minimal role in the absorption of fat digestion products. Intestinal absorption of nutrients is a basic vital mechanism that depends essentially on the function of intestinal mucosa. However, intrinsic and extrinsic neural mechanisms that rely on several redundant loops are involved in immediate and long-term control of the outcome of intestinal function.

The relationship between inflammation-induced neuronal excitability and disrupted motor activity in the guinea pig distal colon

BACKGROUND

Colitis is associated with increased excitability of afterhyperpolarization neurons (AH neurons) and facilitated synaptic transmission in the myenteric plexus. These changes are accompanied by disrupted propulsive motility, particularly in ulcerated regions. This study examined the relationship between myenteric AH neuronal hyperexcitability and disrupted propulsive motility.

METHODS

The voltage-activated Na(+) channel opener veratridine, the intermediate conductance Ca(2+) -activated K(+) channel inhibitor TRAM-34 and the 5-HT(4) receptor agonist tegaserod were used to evaluate the effects of neuronal hyperexcitability and synaptic facilitation on propulsive motility in normal guinea pig distal colon. Because trinitrobenzene sulfonic acid (TNBS)-colitis-induced hyperexcitability of myenteric afferent neurons involves increases in hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel activity, the HCN channel inhibitors Cs(+) and ZD7288 were used to suppress AH neuronal activity in TNBS-colitis.

KEY RESULTS

In non-inflamed preparations, veratridine halted propulsive motility (P<0.001). The rate of propulsive motor activity was significantly reduced following addition of TRAM-34 and tegaserod (P<0.001). In TNBS-inflamed preparations, in which motility was temporarily halted or obstructed at sites of ulceration, both Cs(+) and ZD7288 normalized motility through the inflamed regions. Immunohistochemistry studies demonstrated that the proportion of AH neurons in the myenteric plexus was unchanged in ulcerated regions, but there was a 10% reduction in total number of neurons per ganglion.

CONCLUSIONS AND INFERENCES

These findings support the concept that inflammation-induced neuroplasticity in myenteric neurons, involving changes in ion channel activity that lead to enhanced AH neuronal excitability, can contribute to impaired propulsive colonic motility.

Modulation of electrogenic transport processes in the porcine proximal colon by enteric neurotransmitters

The aim of our study was to evaluate the involvement of essential pro- and antisecretory neurotransmitters in regulation of secretion in porcine proximal colon. Choline acetyltransferase (ChAT), nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), substance P (SP), somatostatin (SOM) and neuropeptide Y (NPY) were located immunohistochemically in the epithelium and subepithelial layer. Modulation of epithelial secretion was studied in Ussing chambers. Application of carbachol (CA), sodium nitroprussid (SNP), VIP and SP but not of NPY or SOM resulted in a chloride dependent increase in short circuit current (I(sc) ). I(sc) increase induced by CA, VIP or SNP was not altered by preincubation with tetrodotoxin or indomethacin. In contrast, SP-induced I(sc) increase was diminished by preincubation with tetrodotoxin, indomethacin, L-nitro-arginin-methyl-ester, and atropine but not hexamethonium. Simultaneous application of CA and VIP, or CA and SNP increased the I(sc) stronger as expected. Applying SP/CA led to a smaller increase in I(sc) as calculated. It is concluded that mainly prosecretory neurotransmitters are involved in regulation of colonic secretion. Cross-potentiations of acetylcholine and nitric oxide and acetylcholine and VIP suggest activation of different intracellular cascades. Similar intracellular pathways may be stimulated by acetylcholine and SP, thus preventing an additive effect of the transmitters.

Quantification and morphometry of myenteric neurones in the jejunum of Holtzman rats (Rattus norvegicus)

With 2 figures and 3 tables

SUMMARY

The morphological pattern of the myenteric plexus (MP) is species-specific, and little is known about this pattern in Holtzman rats. The aim of the current experiment was the morphological and quantitative study of myenteric neurones in the Holtzman rat jejunum. Hematoxylin-Eosin and NADH-diaphorase (NADH-dp) staining were used to assess muscular layer thickness, neurone cell body area (CBA) and nuclei area (NA). Muscular layer thickness was found to be 114.77 ± 14.89 μm. Neuronal densities across the subregions of the jejunum were similar: mesenteric, 11.78 ± 2.89/mm(2) ; intermediate, 12.06 ± 2.69/mm(2) ; and antimesenteric, 10.67 ± 1.89/mm(2) . As expected, there was positive correlation between the CBA and NA of 79.19, 79.26 and 78.5% in the mesenteric, intermediate and antimesenteric subregions of the jejunum, respectively. Medium-sized neurones predominated in the ganglionic arrangement of the MP. These results indicate that the NADH-dp myenteric neurones in the jejunum of Holtzman rats are similar in many aspects to those found in the ileum of Holtzman rats and to those found in the small intestine of Wistar rats, including their location, ganglionic disposition and predominance of medium-sized CBA. However, neuronal density in the jejunum is lower than in the ileum. Based on these results showing morphological similarities to the MP of the Wistar rat, the Holtzman strain can be used to investigate the effects of adverse conditions on the morphology of the MP.

Multiple neural oscillators and muscle feedback are required for the intestinal fed state motor program

After a meal, the gastrointestinal tract exhibits a set of behaviours known as the fed state. A major feature of the fed state is a little understood motor pattern known as segmentation, which is essential for digestion and nutrient absorption. Segmentation manifests as rhythmic local constrictions that do not propagate along the intestine. In guinea-pig jejunum in vitro segmentation constrictions occur in short bursts together with other motor patterns in episodes of activity lasting 40-60 s and separated by quiescent episodes lasting 40-200 s. This activity is induced by luminal nutrients and abolished by blocking activity in the enteric nervous system (ENS). We investigated the enteric circuits that regulate segmentation focusing on a central feature of the ENS: a recurrent excitatory network of intrinsic sensory neurons (ISNs) which are characterized by prolonged after-hyperpolarizing potentials (AHPs) following their action potentials. We first examined the effects of depressing AHPs with blockers of the underlying channels (TRAM-34 and clotrimazole) on motor patterns induced in guinea-pig jejunum, in vitro, by luminal decanoic acid. Contractile episode durations increased markedly, but the frequency and number of constrictions within segmenting bursts and quiescent period durations were unaffected. We used these observations to develop a computational model of activity in ISNs, excitatory and inhibitory motor neurons and the muscle. The model predicted that: i) feedback to ISNs from contractions in the circular muscle is required to produce alternating activity and quiescence with the right durations; ii) transmission from ISNs to excitatory motor neurons is via fast excitatory synaptic potentials (EPSPs) and to inhibitory motor neurons via slow EPSPs. We conclude that two rhythm generators regulate segmentation: one drives contractions within segmentation bursts, the other the occurrence of bursts. The latter depends on AHPs in ISNs and feedback to these neurons from contraction of the circular muscle.

Activation of corticotropin-releasing factor receptor 2 mediates the colonic motor coping response to acute stress in rodents

BACKGROUND & AIMS

Corticotropin-releasing factor receptor-1 (CRF(1)) mediates the stress-induced colonic motor activity. Less is known about the role of CRF(2) in the colonic response to stress.

METHODS

We studied colonic contractile activity in rats and CRF(2)-/-, CRF-overexpressing, and wild-type mice using still manometry; we analyzed defecation induced by acute partial-restraint stress (PRS), and/or intraperitoneal injection of CRF ligands. In rats, we monitored activation of the colonic longitudinal muscle myenteric plexus (LMMP) neurons and localization of CRF(1) and CRF(2) using immunohistochemical and immunoblot analyses. We measured phosphorylation of extracellular signal-regulated kinase 1/2 by CRF ligands in primary cultures of LMMP neurons (PC-LMMPn) and cyclic adenosine monophosphate (cAMP) production in human embryonic kidney-293 cells transfected with CRF(1) and/or CRF(2).

RESULTS

In rats, a selective agonist of CRF(2) (urocortin 2) reduced CRF-induced defecation (>50%), colonic contractile activity, and Fos expression in the colonic LMMP. A selective antagonist of CRF(2) (astressin(2)-B) increased these responses. Urocortin 2 reduced PRS-induced colonic contractile activity in wild-type and CRF-overexpressing mice, whereas disruption of CRF(2) increased PRS-induced colonic contractile activity and CRF-induced defecation. CRF(2) colocalized with CRF(1) and neuronal nitric oxide synthase in the rat colon, LMMP, and PC-LMMPn. CRF-induced phosphorylation of extracellular signal-regulated kinase in PC-LMMPn; this was inhibited or increased by a selective antagonist of CRF(1) (NBI35965) or astressin(2)-B, respectively. The half maximal effective concentration, EC(50), for the CRF-induced cAMP response was 8.6 nmol/L in human embryonic kidney-293 cells that express only CRF(1); this response was suppressed 10-fold in cells that express CRF(1) and CRF(2).

CONCLUSIONS

In colon tissues of rodents, CRF(2) activation inhibits CRF(1) signaling in myenteric neurons and the stress-induced colonic motor responses. Disruption of CRF(2) function impairs colonic coping responses to stress.

An immunohistochemical study of the distribution of nitric oxide synthase-immunoreactive neurons and fibers in the reticular groove of suckling lambs

The reticular groove (RG) is a specialized region of ruminant forestomach which, in suckling animals, via a vagovagal reflex, transforms itself into a tube to ensure the direct transport of milk from the esophagus to the abomasum. The nervous mechanism controlling the RG movement is not fully understood; however, at this level, the enteric nervous system (ENS) shows the highest neuronal density when compared with other forestomach compartments. Because nitric oxide is considered the putative major mediator of non-adrenergic non-cholinergic smooth muscle relaxation, the aim of the present study was to investigate the ENS of the RG of suckling lambs, both in the floor and in the lip, with particular regard to nitric oxide synthase (NOS)-immunoreactivity (-IR), by means of double immunohistochemical staining. NOS antiserum was used in association with some neurochemical markers which have been utilized by many authors in ENS. A rich innervation of fibers extended along the entire length of the RG. Proceeding distally, the number of neurons stained with a pan-neuronal marker increased; they were more numerous in the lips and lip-floor junction than in the floor itself. However, the percentage of NOS-IR neurons was the same in the proximal and distal parts. Many NOS-IR neurons often co-expressed galanin and dopamine β-hydroxylase. Neurochemical markers, such as calbindin, calcitonin gene-related peptide, IB4 and neurofilament 200 kDa, usually used to identify primary sensory neurons were not expressed in RG neurons, and the co-localization of NOS with tyrosine hydroxylase and substance P was rarely found. When compared with other districts, the RG showed some peculiar aspects, such as the lack of both neurons in the submucosal plexus and the lack of typical sensory neurons.