Multiple mechanisms of fast excitatory synaptic transmission in the enteric nervous system

Short- and Long-Term Effects of Cocaine on Enteric Neuronal Functions

Cocaine is one of the most consumed illegal drugs among (young) adults in the European Union and it exerts various acute and chronic negative effects on psychical and physical health. The central mechanism through which cocaine initially leads to improved performance, followed by addictive behavior, has already been intensively studied and includes effects on the homeostasis of the neurotransmitters dopamine, partly mediated via nicotinic acetylcholine receptors, and serotonin. However, effects on the peripheral nervous system, including the enteric nervous system, are much less understood, though a correlation between cocaine consumption and gastrointestinal symptoms has been reported. The aim of the present study was to gain more information on the effects of cocaine on enteric neuronal functions and the underlying mechanisms. For this purpose, functional experiments using an organ bath, Ussing chamber and neuroimaging techniques were conducted on gastrointestinal tissues from guinea pigs. Key results obtained are that cocaine (1) exhibits a stimulating, non-neuronal effect on gastric antrum motility, (2) acutely (but not chronically) diminishes responses of primary cultured enteric neurons to nicotinic and serotonergic stimulation and (3) reversibly attenuates neuronal-mediated intestinal mucosal secretion. It can be concluded that cocaine, among its central effects, also alters enteric neuronal functions, providing potential explanations for the coexistence of cocaine abuse and gastrointestinal complaints.

The Enteric Glia and Its Modulation by the Endocannabinoid System, a New Target for Cannabinoid-Based Nutraceuticals?

The enteric nervous system (ENS) is a part of the autonomic nervous system that intrinsically innervates the gastrointestinal (GI) tract. Whereas enteric neurons have been deeply studied, the enteric glial cells (EGCs) have received less attention. However, these are immune-competent cells that contribute to the maintenance of the GI tract homeostasis through supporting epithelial integrity, providing neuroprotection, and influencing the GI motor function and sensation. The endogenous cannabinoid system (ECS) includes endogenous classical cannabinoids (anandamide, 2-arachidonoylglycerol), cannabinoid-like ligands (oleoylethanolamide (OEA) and palmitoylethanolamide (PEA)), enzymes involved in their metabolism (FAAH, MAGL, COX-2) and classical (CB1 and CB2) and non-classical (TRPV1, GPR55, PPAR) receptors. The ECS participates in many processes crucial for the proper functioning of the GI tract, in which the EGCs are involved. Thus, the modulation of the EGCs through the ECS might be beneficial to treat some dysfunctions of the GI tract. This review explores the role of EGCs and ECS on the GI tract functions and dysfunctions, and the current knowledge about how EGCs may be modulated by the ECS components, as possible new targets for cannabinoids and cannabinoid-like molecules, particularly those with potential nutraceutical use.

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.

Computational simulations and Ca2+ imaging reveal that slow synaptic depolarizations (slow EPSPs) inhibit fast EPSP evoked action potentials for most of their time course in enteric neurons

Transmission between neurons in the extensive enteric neural networks of the gut involves synaptic potentials with vastly different time courses and underlying conductances. Most enteric neurons exhibit fast excitatory post-synaptic potentials (EPSPs) lasting 20–50 ms, but many also exhibit slow EPSPs that last up to 100 s. When large enough, slow EPSPs excite action potentials at the start of the slow depolarization, but how they affect action potentials evoked by fast EPSPs is unknown. Furthermore, two other sources of synaptic depolarization probably occur in enteric circuits, activated via GABA_A or GABA_C receptors; how these interact with other synaptic depolarizations is also unclear. We built a compartmental model of enteric neurons incorporating realistic voltage-dependent ion channels, then simulated fast EPSPs, slow EPSPs and GABA_A or GABA_C ligand-gated Cl^- channels to explore these interactions. Model predictions were tested by imaging Ca²⁺ transients in myenteric neurons ex vivo as an indicator of their activity during synaptic interactions. The model could mimic firing of myenteric neurons in mouse colon evoked by depolarizing current during intracellular recording and the fast and slow EPSPs in these neurons. Subthreshold fast EPSPs evoked spikes during the rising phase of a slow EPSP, but suprathreshold fast EPSPs could not evoke spikes later in a slow EPSP. This predicted inhibition was confirmed by Ca²⁺ imaging in which stimuli that evoke slow EPSPs suppressed activity evoked by fast EPSPs in many myenteric neurons. The model also predicted that synchronous activation of GABA_A receptors and fast EPSPs potentiated firing evoked by the latter, while synchronous activation of GABA_C receptors with fast EPSPs, potentiated firing and then suppressed it. The results reveal that so-called slow EPSPs have a biphasic effect being likely to suppress fast EPSP evoked firing over very long periods, perhaps accounting for prolonged quiescent periods seen in enteric motor patterns.

The gastrointestinal tract is the only organ with an extensive semi-autonomous nervous system that generates complex contraction patterns independently. Communication between neurons in this “enteric” nervous system is via depolarizing synaptic events with dramatically different time courses including fast synaptic potentials lasting around 20–50 ms and slow depolarizing synaptic potentials lasting for 10–120 s. Most neurons have both. We explored how slow synaptic depolarizations affect generation of action potentials by fast synaptic potentials using computational simulation of small networks of neurons implemented as compartmental models with realistic membrane ion channels. We found that slow synaptic depolarizations have biphasic effects; they initially make fast synaptic potentials more likely to trigger action potentials, but then actually prevent action potential generation by fast synaptic potentials with the inhibition lasting several 10s of seconds. We confirmed the inhibitory effects of the slow synaptic depolarizations using live Ca²⁺ imaging of enteric neurons from mouse colon in isolated tissue. Our results identify a novel form of synaptic inhibition in the enteric nervous system of the gut, which may account for the vastly differing time courses between signalling in individual gut neurons and rhythmic contractile patterns that often repeat at more than 60 s intervals.

Modeling Intestinal Stem Cell Function with Organoids

Intestinal epithelial cells (IECs) are crucial for the digestive process and nutrient absorption. The intestinal epithelium is composed of the different cell types of the small intestine (mainly, enterocytes, goblet cells, Paneth cells, enteroendocrine cells, and tuft cells). The small intestine is characterized by the presence of crypt-villus units that are in a state of homeostatic cell turnover. Organoid technology enables an efficient expansion of intestinal epithelial tissue in vitro. Thus, organoids hold great promise for use in medical research and in the development of new treatments. At present, the cholinergic system involved in IECs and intestinal stem cells (ISCs) are attracting a great deal of attention. Thus, understanding the biological processes triggered by epithelial cholinergic activation by acetylcholine (ACh), which is produced and released from neuronal and/or non-neuronal tissue, is of key importance. Cholinergic signaling via ACh receptors plays a pivotal role in IEC growth and differentiation. Here, we discuss current views on neuronal innervation and non-neuronal control of the small intestinal crypts and their impact on ISC proliferation, differentiation, and maintenance. Since technology using intestinal organoid culture systems is advancing, we also outline an organoid-based organ replacement approach for intestinal diseases.

Circuit-specific enteric glia regulate intestinal motor neurocircuits

Glia in the central nervous system exert precise spatial and temporal regulation over neural circuitry on a synapse-specific basis, but it is unclear if peripheral glia share this exquisite capacity to sense and modulate circuit activity. In the enteric nervous system (ENS), glia control gastrointestinal motility through bidirectional communication with surrounding neurons. We combined glial chemogenetics with genetically encoded calcium indicators expressed in enteric neurons and glia to study network-level activity in the intact myenteric plexus of the proximal colon. Stimulation of neural fiber tracts projecting in aboral, oral, and circumferential directions activated distinct populations of enteric glia. The majority of glia responded to both oral and aboral stimulation and circumferential pathways, while smaller subpopulations were activated only by ascending and descending pathways. Cholinergic signaling functionally specifies glia to the descending circuitry, and this network plays an important role in repressing the activity of descending neural pathways, with some degree of cross-inhibition imposed upon the ascending pathway. Glial recruitment by purinergic signaling functions to enhance activity within ascending circuit pathways and constrain activity within descending networks. Pharmacological manipulation of glial purinergic and cholinergic signaling differentially altered neuronal responses in these circuits in a sex-dependent manner. Collectively, our findings establish that the balance between purinergic and cholinergic signaling may differentially control specific circuit activity through selective signaling between networks of enteric neurons and glia. Thus, enteric glia regulate the ENS circuitry in a network-specific manner, providing profound insights into the functional breadth and versatility of peripheral glia.

Synaptic Components, Function and Modulation Characterized by GCaMP6f Ca2+ Imaging in Mouse Cholinergic Myenteric Ganglion Neurons

The peristaltic contraction and relaxation of intestinal circular and longitudinal smooth muscles is controlled by synaptic circuit elements that impinge upon phenotypically diverse neurons in the myenteric plexus. While electrophysiological studies provide useful information concerning the properties of such synaptic circuits, they typically involve tissue disruption and do not correlate circuit activity with biochemically defined neuronal phenotypes. To overcome these limitations, mice were engineered to express the sensitive, fast Ca²⁺ indicator GCaMP6f selectively in neurons that express the acetylcholine (ACh) biosynthetic enzyme choline acetyltransfarse (ChAT) thereby allowing rapid activity-driven changes in Ca²⁺ fluorescence to be observed without disrupting intrinsic connections, solely in cholinergic myenteric ganglion (MG) neurons. Experiments with selective receptor agonists and antagonists reveal that most mouse colonic cholinergic (i.e., GCaMP6f⁺/ChAT⁺) MG neurons express nicotinic ACh receptors (nAChRs), particularly the ganglionic subtype containing α3 and β4 subunits, and most express ionotropic serotonin receptors (5-HT₃Rs). Cholinergic MG neurons also display small, spontaneous Ca²⁺ transients occurring at ≈ 0.2 Hz. Experiments with inhibitors of Na⁺ channel dependent impulses, presynaptic Ca²⁺ channels and postsynaptic receptor function reveal that the Ca²⁺ transients arise from impulse-driven presynaptic activity and subsequent activation of postsynaptic nAChRs or 5-HT₃Rs. Electrical stimulation of axonal connectives to MG evoked Ca²⁺ responses in the neurons that similarly depended on nAChRs or/and 5-HT₃Rs. Responses to single connective shocks had peak amplitudes and rise and decay times that were indistinguishable from the spontaneous Ca²⁺ transients and the largest fraction had brief synaptic delays consistent with activation by monosynaptic inputs. These results indicate that the spontaneous Ca²⁺ transients and stimulus evoked Ca²⁺ responses in MG neurons originate in circuits involving fast chemical synaptic transmission mediated by nAChRs or/and 5-HT₃Rs. Experiments with an α7-nAChR agonist and antagonist, and with pituitary adenylate cyclase activating polypeptide (PACAP) reveal that the same synaptic circuits display extensive capacity for presynaptic modulation. Our use of non-invasive GCaMP6f/ChAT Ca²⁺ imaging in colon segments with intrinsic connections preserved, reveals an abundance of direct and modulatory synaptic influences on cholinergic MG neurons.

Muscarinic receptor M3 contributes to intestinal stem cell maintenance via EphB/ephrin-B signaling

Acetylcholine (ACh) signaling through activation of nicotinic and muscarinic ACh receptors regulates expression of specific genes that mediate and sustain proliferation, differentiation, and homeostasis in the intestinal crypts. This signaling plays a pivotal role in the regulation of intestinal stem cell function, but the details have not been clarified. Here, we performed experiments using type 3 muscarinic acetylcholine receptor (M3) knockout mice and their intestinal organoids and report that endogenous ACh affects the size of the intestinal stem niche via M3 signaling. RNA sequencing of crypts identified up-regulation of the EphB/ephrin-B signaling pathway. Furthermore, using an MEK inhibitor (U0126), we found that mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling, which is downstream of EphB/ephrin-B signaling, is activated in M3-deficient crypts. Collectively, M3, EphB/ephrin-B, and the MAPK/ERK signaling cascade work together to maintain the homeostasis of intestinal epithelial cell growth and differentiation following modifications of the cholinergic intestinal niche.

Nutraceuticals and Enteric Glial Cells

Until recently, glia were considered to be a structural support for neurons, however further investigations showed that glial cells are equally as important as neurons. Among many different types of glia, enteric glial cells (EGCs) found in the gastrointestinal tract, have been significantly underestimated, but proved to play an essential role in neuroprotection, immune system modulation and many other functions. They are also said to be remarkably altered in different physiopathological conditions. A nutraceutical is defined as any food substance or part of a food that provides medical or health benefits, including prevention and treatment of the disease. Following the description of these interesting peripheral glial cells and highlighting their role in physiological and pathological changes, this article reviews all the studies on the effects of nutraceuticals as modulators of their functions. Currently there are only a few studies available concerning the effects of nutraceuticals on EGCs. Most of them evaluated molecules with antioxidant properties in systemic conditions, whereas only a few studies have been performed using models of gastrointestinal disorders. Despite the scarcity of studies on the topic, all agree that nutraceuticals have the potential to be an interesting alternative in the prevention and/or treatment of enteric gliopathies (of systemic or local etiology) and their associated gastrointestinal conditions.

Neuronal innervation of the intestinal crypt

Mucosal damage is a key feature of inflammatory bowel diseases (IBD) and healing of the mucosa is an endpoint of IBD treatment that is often difficult to achieve. Autonomic neurons of the parasympathetic and sympathetic nervous system may influence intestinal epithelial cell growth and modulating epithelial innervation could for that reason serve as an interesting therapeutic option to improve mucosal healing. Understanding of the biological processes triggered by nonspecific and specific epithelial adrenergic and cholinergic receptor activation is of key importance. At present, with rising technological advances, bioelectronic neuromodulation as treatment modality has gained momentum. We discuss the current view on state-of-the-art innervation of the intestinal crypt and its impact on epithelial cell growth and differentiation. Furthermore, we outline bioelectronic technology and review its relevance to wound healing processes.

Effect of Tityus serrulatus scorpion venom on isolated jejunum: A very useful tool to study the interaction between neurons in the enteric nervous system

Scorpion envenomation is a public health problem in tropical and subtropical areas. In Brazil, Tityus serrulatus is the biggest cause of accidents with venomous animals. Tityus serrulatus venom causes symptoms related to a great activation of the autonomic system attributed to a massive release of sympathetic and parasympathetic mediators. This effect is attributed to the presence of toxins acting in Na⁺ and K⁺ ion channels, leading to an increase in cell excitability. Although gastrointestinal symptoms, like diarrhoea and sialorrhea, is observed in moderate to severe cases, little attention is given in clinical reports. Gastrointestinal motility is controlled by the enteric nervous system which is composed of a wide variety of interconnected neurons that are influenced by the sympathetic and parasympathetic nervous systems. Thus, this work aimed to characterize the effects of Tityus serrulatus venom on sympathetic and parasympathetic neurotransmission of rat jejunum, as well as to investigate possibles effects on other neurons of the enteric nervous system. To this, we verify the effects of Tityus serrulatus venom on the contractility of isolated rat jejunum through organ-bath experiments. We observed that venom can induce both contraction and relaxation. The contraction was partially inhibited by atropine (1 μM) and by suramin (0.1 mM) through tetrodotoxin-resistant and sensitive mechanisms. The relaxation was completely inhibited by 3 μM propranolol and partially inhibited by 1 μM phentolamine. Suramin induced a slowing of relaxation curve. Tetrodotoxin completely inhibits the relaxation induced by Tityus serrulatus venom, but the contraction curves were only partially reduced in their initial portion. The final part of the curve was largely enhanced by Tetrodotoxin. Atropine blocks almost completely the contraction curve in the presence of Tetrodotoxin. These results indicate that Tityus serrulatus venom induces the release of both excitatory (predominantly acetylcholine) and inhibitory (mainly noradrenaline) neurotransmitters. The effects of Tityus serrulatus venom on organ contractility was quite complex and seem to derive from a diffuse and nonspecific release of mediators from autonomic and enteric nervous systems. Further investigation of venom action and its isolated toxins can reveal important aspects to deepen our knowledge about the enteric nervous system transmission and the interaction between excitatory and inhibitory mediators as well as the physiological role of Na⁺ and K⁺ ion channels in gut motility.

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

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

Gastroprokinetic agent, mosapride inhibits 5-HT3 receptor currents in NCB-20 cells

Mosapride accelerates gastric emptying by acting on 5-hydroxytryptamine type 4 (5-HT₄) receptor and is frequently used in the treatment of gastrointestinal (GI) disorders requiring gastroprokinetic efficacy. We tested the effect of mosapride on 5-hydroxytryptamine type 3 (5-HT₃) receptor currents because the 5-HT₃ receptors are also known to be expressed in the GI system and have an important role in the regulation of GI functions. Using the whole-cell voltage clamp method, we compared the currents of the 5-HT₃ receptors when 5-HT was applied alone or was co-applied with mosapride in cultured NCB-20 cells known to express 5-HT₃ receptors. The 5-HT₃ receptor current amplitudes were inhibited by mosapride in a concentration-dependent manner. Mosapride blocked the peak currents evoked by the application of 5-HT in a competitive manner because the EC₅₀ shifted to the right without changing the maximal effect. The rise slopes of 5-HT₃ receptor currents were decreased by mosapride. Pre-application of mosapride before co-application, augmented the inhibitory effect of mosapride, which suggests a closed channel blocking mechanism. Mosapride also blocked the opened 5-HT₃ receptor because it inhibited the 5-HT₃ receptor current in the middle of the application of 5-HT. It accelerated desensitization of the 5-HT₃ receptor but did not change the recovery process from the receptor desensitization. There were no voltage-, or use-dependency in its blocking effects. These results suggest that mosapride inhibited the 5-HT₃ receptor through a competitive blocking mechanism probably by binding to the receptor in closed state, which could be involved in the pharmacological effects of mosapride to treat GI disorders.

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.

Is There a Role for Endogenous 5-HT in Gastrointestinal Motility? How Recent Studies Have Changed Our Understanding

Over the past few years, there have been dramatic changes in our understanding of the role of endogenous 5-hydroxytryptamine (5-HT) in the generation of gastrointestinal (GI) motility patterns in the small and large intestine. The idea that endogenous 5-HT played a major role in the generation of peristalsis in the small intestine was first proposed in the mid 1950s, after it was discovered that endogenous 5-HT could be released from the mucosa at a similar time that peristalsis occurred; and that exogenous 5-HT could potently stimulate peristalsis. The fact that exogenous 5-HT stimulated peristalsis and that there was a similarity in timing between the release of 5-HT from the mucosa and the onset of peristalsis led investigators to propose that release of endogenous 5-HT from the mucosa was causally related to the generation of peristalsis. In further support of this, other studies showed that selective 5-HT antagonists could inhibit or block peristalsis, and other motor patterns, such as the migrating motor complex. Taken together, based on these findings, some laboratories believed that endogenous 5-HT (synthesized in the gut wall) was an important mediator, or initiator, of different propulsive motor patterns in the lower GI tract. This notion changed dramatically in the past few years, however, after it was discovered that removal of the mucosa abolished all cyclical release of endogenous 5-HT, but did not block peristalsis, nor the cyclical migrating complex. Furthermore, other laboratories revealed that genetic deletion of the gene tryptophan hydroxylase 1 (TPH-1) (that synthesizes endogenous 5-HT in the mucosa) actually had no inhibitory effect on transit of intestinal contents in live animals. Then, perhaps one of the most startling of all observations was the discovery that selective 5-HT receptor antagonists actually have the same inhibitory effects on peristalsis and the migrating complex in segments of intestine that had been depleted of all endogenous 5-HT. Taken together, these recent findings have led to a major revision in our understanding of the functional role of endogenous 5-HT in the generation of propulsive motor patterns in the lower GI tract. This review will focus on how our understanding of endogenous 5-HT in the GI tract has changed substantially in recent times.

Purinergic Signalling in the Gut

The article will begin with the discovery of purinergic inhibitory neuromuscular transmission in the 1960s/1970s, the proposal for purinergic cotransmission in 1976 and the recognition that sympathetic nerves release adenosine 5'-triphosphate (ATP), noradrenaline and neuropeptide Y, while non-adrenergic, non-cholinergic inhibitory nerve cotransmitters are ATP, nitric oxide and vasoactive intestinal polypeptide in variable proportions in different regions of the gut. Later, purinergic synaptic transmission in the myenteric and submucosal plexuses was established and purinergic receptors expressed by both glial and interstitial cells. The focus will then be on purinergic mechanosensory transduction involving release of ATP from mucosal epithelial cells during distension to activate P2X3 receptors on submucosal sensory nerve endings. The responses of low threshold fibres mediate enteric reflex activity via intrinsic sensory nerves, while high threshold fibres initiate pain via extrinsic sensory nerves. Finally, the involvement of purinergic signalling in an animal model of colitis will be presented, showing that during distension there is increased ATP release, increased P2X3 receptor expression on calcitonin gene-related peptide-labelled sensory neurons and increased sensory nerve activity.

Submucosal neurons and enteric glial cells expressing the P2X7 receptor in rat experimental colitis

The aim of this study was to evaluate the effect of ulcerative colitis on the submucosal neurons and glial cells of the submucosal ganglia of rats. 2,4,6-Trinitrobenzene sulfonic acid (TNBS; colitis group) was administered in the colon to induce ulcerative colitis, and distal colons were collected after 24h. The colitis rats were compared with those in the sham and control groups. Double labelling of the P2X7 receptor with calbindin (marker for intrinsic primary afferent neurons, IPANs, submucosal plexus), calretinin (marker for secretory and vasodilator neurons of the submucosal plexus), HuC/D and S100β was performed in the submucosal plexus. The density (neurons per area) of submucosal neurons positive for the P2X7 receptor, calbindin, calretinin and HuC/D decreased by 21%, 34%, 8.2% and 28%, respectively, in the treated group. In addition, the density of enteric glial cells in the submucosal plexus decreased by 33%. The profile areas of calbindin-immunoreactive neurons decreased by 25%. Histological analysis revealed increased lamina propria and decreased collagen in the colitis group. This study demonstrated that ulcerative colitis affected secretory and vasodilatory neurons, IPANs and enteric glia of the submucosal plexus expressing the P2X7 receptor.

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.

Anti-Hu antibodies activate enteric and sensory neurons

IgG of type 1 anti-neuronal nuclear antibody (ANNA-1, anti-Hu) specificity is a serological marker of paraneoplastic neurological autoimmunity (including enteric/autonomic) usually related to small-cell lung carcinoma. We show here that IgG isolated from such sera and also affinity-purified anti-HuD label enteric neurons and cause an immediate spike discharge in enteric and visceral sensory neurons. Both labelling and activation of enteric neurons was prevented by preincubation with the HuD antigen. Activation of enteric neurons was inhibited by the nicotinic receptor antagonists hexamethonium and dihydro-β-erythroidine and reduced by the P2X antagonist pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid (PPADS) but not by the 5-HT₃ antagonist tropisetron or the N-type Ca-channel blocker ω-Conotoxin GVIA. Ca⁺⁺ imaging experiments confirmed activation of enteric neurons but not enteric glia. These findings demonstrate a direct excitatory action of ANNA-1, in particular anti-HuD, on visceral sensory and enteric neurons, which involves nicotinic and P2X receptors. The results provide evidence for a novel link between nerve activation and symptom generation in patients with antibody-mediated gut dysfunction.

Immunolocalization of AMPA receptor subunits within the enteric nervous system of the mouse colon and the effect of their activation on spontaneous colonic contractions

BACKGROUND

The appropriate expression of specific neurotransmitter receptors within the cellular networks that compose the enteric nervous system (ENS) is central to the regulation of gastrointestinal (GI) functions. While the ENS expression patterns of the neurotransmitter glutamate have been well documented, the localization of its receptors on ENS neurons remains to be fully characterized. We investigated the expression patterns of glutamate receptor AMPA subunits within ENS neurons of the mouse colon and the consequences of their pharmacological activation on spontaneous colonic contractility.

METHODS

RT-PCR was used to detect individual AMPA receptor (GluR 1-4) subunit expression at the mRNA level in mouse colon tissue. Immunohistochemistry and confocal microscopy was used to localize the expression of the GluR1 and 4 subunits in colon tissue. Brain tissue was used as a positive control. Organ bath preparations were used to determine the effect of AMPA receptors activation on the force and frequency of colonic longitudinal smooth muscle spontaneous contractions.

KEY RESULTS

GluR1, 3, 4 mRNA was detected in the mouse colon. Immunoreactivity for GluR1 and 4 subunits was detected on the somatic and dendritic surfaces of subpopulations of neurochemically defined ENS neurons. The pharmacological activation of AMPA receptors increased the force but not frequency of spontaneous colonic contractions.

CONCLUSIONS & INFERENCES

Molecularly distinct AMPA receptor subtypes are differentially expressed within the neural networks of the mouse colon and have a direct role in motility. These data provide the rationale for the development of AMPA-selective ligands for the therapeutic delivery to the GIT in motility disorders.

Enhanced Sensitivity of α3β4 Nicotinic Receptors in Enteric Neurons after Long-Term Morphine: Implication for Opioid-Induced Constipation

Opioid-induced constipation is a major side effect that persists with long-term opioid use. Previous studies demonstrated that nicotine-induced contractions are enhanced after long-term morphine exposure in guinea pig ileum. In the present study, we examined whether the increased sensitivity to nicotine could be observed in single enteric neurons after long-term morphine exposure, determined the subunits in mouse enteric neurons, and examined the effect of nicotine in reversing opioid-induced constipation. Nicotine (0.03-1 mM) dose-dependently induced inward currents from a holding potential of -60 mV in isolated single enteric neurons from the mouse ileum. The amplitude of the currents, but not the potency to nicotine, was significantly increased in neurons receiving long-term (16-24 h) but not short-term (10 min) exposure to morphine. Quantitative mRNA analysis showed that nicotinic acetylcholine receptor (nAChR) subunit expression in the mouse ileum was α3 ≥ β2 > β4 > α5 > α4 > β3 > α6. Nicotine-induced currents were obtained in neurons from α7, β2, α5, and α6 knockout mice. The currents were, however, inhibited by mecamylamine (10 μM) and the α3β4 blocker α-conotoxin AuIB (3 μM), suggesting that nicotine-induced currents were mediated by the α3β4 subtype of nAChRs on enteric neurons. Conversely, NS3861, a partial agonist at α3β4 nAChR, enhanced fecal pellet expulsion in a dose-dependent manner in mice that received long-term, but not short-term, morphine treatment. Overall, our findings suggest that the efficacy of nAChR agonists on enteric neurons is enhanced after long-term morphine exposure, and activation of the α3β4 subtype of nAChR reverses chronic, but not acute, morphine-induced constipation.

The Role of 5-HT3 Receptors in Signaling from Taste Buds to Nerves

Activation of taste buds triggers the release of several neurotransmitters, including ATP and serotonin (5-hydroxytryptamine; 5-HT). Type III taste cells release 5-HT directly in response to acidic (sour) stimuli and indirectly in response to bitter and sweet tasting stimuli. Although ATP is necessary for activation of nerve fibers for all taste stimuli, the role of 5-HT is unclear. We investigated whether gustatory afferents express functional 5-HT3 receptors and, if so, whether these receptors play a role in transmission of taste information from taste buds to nerves. In mice expressing GFP under the control of the 5-HT(3A) promoter, a subset of cells in the geniculate ganglion and nerve fibers in taste buds are GFP-positive. RT-PCR and in situ hybridization confirmed the presence of 5-HT(3A) mRNA in the geniculate ganglion. Functional studies show that only those geniculate ganglion cells expressing 5-HT3A-driven GFP respond to 10 μM 5-HT and this response is blocked by 1 μM ondansetron, a 5-HT3 antagonist, and mimicked by application of 10 μM m-chlorophenylbiguanide, a 5-HT3 agonist. Pharmacological blockade of 5-HT3 receptors in vivo or genetic deletion of the 5-HT3 receptors reduces taste nerve responses to acids and other taste stimuli compared with controls, but only when urethane was used as the anesthetic. We find that anesthetic levels of pentobarbital reduce taste nerve responses apparently by blocking the 5-HT3 receptors. Our results suggest that 5-HT released from type III cells activates gustatory nerve fibers via 5-HT3 receptors, accounting for a significant proportion of the neural taste response.

Serotonergic Mechanisms Regulating the GI Tract: Experimental Evidence and Therapeutic Relevance

Serotonin (5-hydroxytryptamine; 5-HT) is best known as a neurotransmitter critical for central nervous system (CNS) development and function. 95% of the body's serotonin, however, is produced in the intestine where it has been increasingly recognized for its hormonal, autocrine, paracrine, and endocrine actions. This chapter provides the most current knowledge of the critical autocrine and paracrine roles of 5-HT in intestinal motility and inflammation as well as its function as a hormone in osteocyte homeostasis. Therapeutic applications in each of these areas are also discussed.

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

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

Impact of ectonucleotidases in autonomic nervous functions

Adenine and uracil nucleotides play key functions in the autonomic nervous system (ANS). For instance, ATP acts as a neurotransmitter, co-transmitter and neuromodulator in the ANS. The purinergic system encompasses (1) receptors that respond to extracellular purines, which are designated as P1 and P2 purinoceptors, (2) purine release and uptake, and (3) a cascade of enzymes that regulate the concentration of purines near the cell surface. Ectonucleotidases and adenosine deaminase (ADA) are enzymes responsible for the hydrolysis of ATP (and other nucleotides such as ADP, UTP, UDP, AMP) and adenosine, respectively. Accordingly, these enzymes are expected to play an important role in the control of neuro-effector transmission in tissues innervated by both the sympathetic and parasympathetic divisions of the ANS. Indeed, ectonucleotidases have the ability to either terminate P2 receptor responses initiated by nucleoside triphosphates (ATP and UTP), and/or to favor the activation of ADP (e.g. P2Y1,12,13) and UDP (e.g. P2Y6) and/or adenosine (P1) specific receptors. In addition, ectonucleotidases can also importantly protect some P2 receptors from desensitization (e.g. P2X1, P2Y1). In this review, we present the (putative) roles of ectonucleotidases and ADA in the ANS with a focus on their regulatory activity at neuro-effector junctions in the following tissues: heart, vas deferens, urinary bladder, salivary glands, blood vessels and the intestine. We also present their implication in nociceptive transmission.

P2X3 receptors and sensitization of autonomic reflexes

A great deal of basic and applied physiology and pharmacology in sensory and autonomic neuroscience has teased apart mechanisms that drive normal perception of mechanical, thermal and chemical signals and convey them to CNS, the distinction of fiber types and receptors and channels that mediate them, and how they may become dysfunctional or maladaptive in disease. Likewise, regulation of efferent autonomic traffic to control organ reflexes has been well studied. In both afferent and efferent limbs, a wide array of potential therapeutic mechanisms has surfaced, some of which have progressed into clinic, if not full regrastration. One conversation that has been less well progressed relates to how the afferent limb and its sensitization shapes the efferent outputs, and where modulation may offer new therapeutic avenues, especially for poorly addressed and common signs and symptoms of disease. Therapeutics for CV disease (HF, hypertension), respiratory disease (asthma, COPD), urological disease (OAB), GI disease (IBS), and inter alia, have largely focused on the efferent control of effector cells to modulate movement, contraction and secretion; medicinal needs remain with limits to efficacy, AEs and treatment resistance being common. We now must turn, in the quest for improved therapeutics, to understand how sensation from these organs becomes maladapted and sensitized in disease, and what opportunities may arise for improved therapeutics given the abundance of targets, many pharmacologically untapped, on the afferent side. One might look at the treatment resistant hypertension and the emerging benefit of renal denervation; or urinary bladder overactivity / neurogenic bladder and the emergence of neuromodulation, capsaicin instillation or botox injections to attenuate sensitized reflexes, as examples of merely the start of such progress. This review examines this topic more deeply, as applies to four major organ systems all sharing a great need from unsatisfied patients.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

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.

Differential effects of experimental ulcerative colitis on P2X7 receptor expression in enteric neurons

The digestive tracts of ulcerative colitis and Crohn's disease patients present with pathophysiological processes and intestinal necrosis. This study examined the P2X7 receptor and changes in the distal colon in enteric neurons of rats with experimental ulcerative colitis. The analysis was performed in the distal colons of rats with ulcerative colitis induced by the administration of 2,4,6-trinitrobenzene sulfonic acid (colitis group). The survival time after colitis induction was 24 h. The treated animals were compared to sham rats injected with phosphate-buffered saline and to animals with no intervention (control group). Tissues were prepared for immunohistochemical double-staining methods to examine P2X7 receptor, choline acetyltransferase (ChAT), calbindin, calretinin, anti-HuC/D (pan-neuronal) and S100β (pan-glial). The colocalization of the P2X7 receptor-immunoreactive (IR) cells was observed in the myenteric plexus with nitric oxide synthase (NOS)-, ChAT-,calbindin-, calretinin- and HuC/D-IR neurons and S100β-IR cells in the control, sham and colitis groups. The neuronal density (cell bodies/cm(2)) decreased in the myenteric plexus by 11, 18, 34, 22 and 60% in the P2X7 receptor, NOS-, ChAT-, calbindin- and calretinin-IR neurons, respectively. In addition, the densities (cell bodies/cm(2)) of HuC/D-IR neurons and S100β-IR enteric glial cells decreased by 33 and 29%, respectively. The profile areas were reduced by 6.8 and 21% in NOS- and ChAT-IR neurons, respectively. There was also a 20% increase of calbindin-IR neurons. Morphological changes were observed, such as increased neutrophils, disintegration of the intestinal epithelium and goblet cells and decreased collagen. This study demonstrated that colitis differentially affects P2X7 receptor-expressing enteric neurons based on their chemical codes and may cause changes in morphology and motility.

Calcium imaging at kHz frame rates resolves millisecond timing in neuronal circuits and varicosities

We have configured a widefield fast imaging system that allows imaging at 1000 frames per second (512x512 pixels). The system was extended with custom processing tools including a time correlation method to facilitate the analysis of static subcellular compartments (e.g. neuronal varicosities) with enhanced contrast, as well as a dynamic intensity processing (DIP) algorithm that aids in data size reduction and fast visualization and interpretation of timing and directionality in neuronal circuits. This system, together with our custom developed processing tools enables efficient detection of fast physiological events, such as action potential dependent calcium steps. We show, using a specific blocker of nerve communication, that with this setup it is possible to discriminate between a pre and post synaptic event in an all optical way.

Acetylcholine serves as a derepressor in Loperamide-induced Opioid-Induced Bowel Dysfunction (OIBD) in zebrafish

The mechanisms underlying gut development, especially peristalsis, are widely studied topics. However, the causes of gut peristalsis-related diseases, especially Opioid-Induced Bowel Dysfunction (OIBD) disorder, have not been well defined. Therefore, our study used zebrafish, a popular model for studying both gut development and peristalsis, and DCFH-DA, a dye that clearly labels the live fish gut lumen, to characterize the formation process of gut lumen as well as the gut movement style in vivo. By applying Loperamide Hydrochloride (LH), the μ-opioid receptor-specific agonist, we established an OIBD-like zebrafish model. Our study found that acetylcholine (ACh) was a key transmitter that derepressed the phenotype induced by LH. Overall, the study showed that the antagonistic role of ACh in the LH-mediated opioid pathway was evolutionarily conserved; moreover, the OIBD-like zebrafish model will be helpful in the future dissection of the molecular pathways involved in gut lumen development and pathology.

Inhibition of native 5-HT3 receptor-evoked contractions in guinea pig and mouse ileum by antimalarial drugs

Quinine, chloroquine and mefloquine are commonly used to treat malaria, however, with associated gastrointestinal (GI) side-effects. These drugs act as antagonists at recombinant 5-HT3 receptors and modulate gut peristalsis. These gastrointestinal side effects may be the result of antagonism at intestinal 5-HT3 receptors. Ileum from male C57BL/6 mice and guinea pigs was mounted longitudinally in organ baths. The concentration-response curves for 5-HT and the selective 5-HT3 agonist 2-Me-5-HT were obtained with 5-HT (pEC50 = 7.57 ± 0.33, 12) more potent (P = 0.004) than 2-Me-5-HT (pEC50 = 5.45 ± 0.58, n = 5) in mouse ileum. There was no difference in potency of 5-HT (pEC50 = 5.42 ± 0.15, n = 8) and 2-Me-5-HT (pIC50 = 5.01 ± 0.55, n = 11) in guinea pig ileum (P > 0.05). Quinine, chloroquine or mefloquine was applied for 10 min and inhibitions prior to submaximal agonist application. In mouse ileum, quinine, chloroquine and mefloquine antagonised 5-HT-induced contractions (pIC50 = 4.9 ± 0.17, n = 7; 4.76 ± 0.14, n = 5; 6.21 ± 0.2, n = 4, correspondingly) with mefloquine most potent (P < 0.05). Quinine, chloroquine and mefloquine antagonised 2-me-5-HT-induced contractions (pIC50 = 6.35 ± 0.11, n = 8; 4.64 ± 0.2, n = 7; 5.11 ± 0.22, n = 6, correspondingly) with quinine most potent (P < 0.05). In guinea-pig ileum, quinine, chloroquine and mefloquine antagonised 5-HT-induced contractions (pIC50 = 5.02 ± 0.15, n = 6; 4.54 ± 0.1, n = 7; 5.32 ± 0.13, n = 5) and 2-me-5-HT-induced contractions (pIC50 = 4.62 ± 0.25, n = 5; 4.56 ± 0.14, n = 6; 5.67 ± 0.12, n = 4) with chloroquine least potent against 5-HT and mefloquine most potent against 2-me-5-HT (P < 0.05). These results support previous studies identifying anti-malarial drugs as antagonists at recombinant 5-HT3 receptors and may also demonstrate the ability of these drugs to influence native 5-HT3 receptor-evoked contractile responses which may account for their associated GI side-effects.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

Selective coexpression of synaptic proteins, α-synuclein, cysteine string protein-α, synaptophysin, synaptotagmin-1, and synaptobrevin-2 in vesicular acetylcholine transporter-immunoreactive axons in the guinea pig ileum

Parkinson's disease is a neurodegenerative disorder characterized by Lewy bodies and neurites composed mainly of the presynaptic protein α-synuclein. Frequently, Lewy bodies and neurites are identified in the gut of Parkinson's disease patients and may underlie associated gastrointestinal dysfunctions. We recently reported selective expression of α-synuclein in the axons of cholinergic neurons in the guinea pig and human distal gut; however, it is not clear whether α-synuclein expression varies along the gut, nor how closely expression is associated with other synaptic proteins. We used multiple-labeling immunohistochemistry to quantify which neurons in the guinea pig ileum expressed α-synuclein, cysteine string protein-α (CSPα), synaptophysin, synaptotagmin-1, or synaptobrevin-2 in their axons. Among the 10 neurochemically defined axonal populations, a significantly greater proportion of vesicular acetylcholine transporter-immunoreactive (VAChT-IR) varicosities (80% ± 1.7%, n = 4, P < 0.001) contained α-synuclein immunoreactivity, and a significantly greater proportion of α-synuclein-IR axons also contained VAChT immunoreactivity (78% ± 1.3%, n = 4) compared with any of the other nine populations (P < 0.001). Among synaptophysin-, synaptotagmin-1-, synaptobrevin-2-, and CSPα-IR varicosities, 98% ± 0.7%, 96% ± 0.7%, 88% ± 1.6%, and 85% ± 2.9% (n = 4) contained α-synuclein immunoreactivity, respectively. Among α-synuclein-IR varicosities, 96% ± 0.9%, 99% ± 0.6%, 83% ± 1.9%, and 87% ± 2.3% (n = 4) contained synaptophysin-, synaptotagmin-1-, synaptobrevin-2-, and CSPα immunoreactivity, respectively. We report a close association between the expression of α-synuclein and the expression of other synaptic proteins in cholinergic axons in the guinea pig ileum. Selective expression of α-synuclein may relate to the neurotransmitter system utilized and predispose cholinergic enteric neurons to degeneration in Parkinson's disease.

The 5-hydroxytryptamine 4 Receptor Agonist-induced Actions and Enteric Neurogenesis in the Gut

We explored a novel effect of 5-hydroxytryptamine 4 receptor (5-HT4R) agonists in vivo to reconstruct the enteric neural circuitry that mediates a fundamental distal gut reflex. The neural circuit insult was performed in guinea pigs and rats by rectal transection and anastomosis. A 5-HT4R-agonist, mosapride citrate (MOS) applied orally and locally at the anastomotic site for 2 weeks promoted the regeneration of the impaired neural circuit or the recovery of the distal gut reflex. MOS generated neurofilament-, 5-HT4R- and 5-bromo-2'-deoxyuridine-positive cells and formed neural network in the granulation tissue at the anastomosis. Possible neural stem cell markers increased during the same time period. These novel actions by MOS were inhibited by specific 5-HT4R-antagonist such as GR113808 (GR) or SB-207266. The activation of enteric neural 5-HT4R promotes reconstruction of an enteric neural circuit that involves possibly neural stem cells. We also succeeded in forming dense enteric neural networks by MOS in a gut differentiated from mouse embryonic stem cells. GR abolished the formation of enteric neural networks. MOS up-regulated the expression of mRNA of 5-HT4R, and GR abolished this upregulation, suggesting MOS differentiated enteric neural networks, mediated via activation of 5-HT4R. In the small intestine in H-line: Thy1 promoter green fluorescent protein (GFP) mice, we obtained clear 3-dimensional imaging of enteric neurons that were newly generated by oral application of MOS after gut transection and anastomosis. All findings indicate that treatment with 5-HT4R-agonists could be a novel therapy for generating new enteric neurons to rescue aganglionic disorders in the whole gut.

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

Recent studies have shown that endogenous serotonin is not required for colonic peristalsis in vitro, nor gastrointestinal (GI) transit in vivo. However, antagonists of 5-Hydroxytryptamine (5-HT) receptors can inhibit peristalsis and GI-transit in mammals, including humans. This raises the question of how these antagonists inhibit GI-motility and transit, if depletion of endogenous 5-HT does not cause any significant inhibitory changes to either GI-motility or transit? We investigated the mechanism by which 5-HT3 and 5-HT4 antagonists inhibit distension-evoked peristaltic contractions in guinea-pig distal colon. In control animals, repetitive peristaltic contractions of the circular muscle were evoked in response to fixed fecal pellet distension. Distension-evoked peristaltic contractions were unaffected in animals with mucosa and submucosal plexus removed, that were also treated with reserpine (to deplete neuronal 5-HT). In control animals, peristaltic contractions were blocked temporarily by ondansetron (1–10 μM) and SDZ-205–557 (1–10 μM) in many animals. Interestingly, after this temporary blockade, and whilst in the continued presence of these antagonists, peristaltic contractions recovered, with characteristics no different from controls. Surprisingly, similar effects were seen in mucosa-free preparations, which had no detectable 5-HT, as detected by mass spectrometry. In summary, distension-evoked peristaltic reflex contractions of the circular muscle layer of the guinea-pig colon can be inhibited temporarily, or permanently, in the same preparation by selective 5-HT3 and 5-HT4 antagonists, depending on the concentration of the antagonists applied. These effects also occur in preparations that lack any detectable 5-HT. We suggest caution should be exercised when interpreting the effects of 5-HT3 and 5-HT4 antagonists; and the role of endogenous 5-HT, in the generation of distension-evoked colonic peristalsis.

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.

Structure activity relationship of synaptic and junctional neurotransmission

Chemical neurotransmission may include transmission to local or remote sites. Locally, contact between 'bare' portions of the bulbous nerve terminal termed a varicosity and the effector cell may be in the form of either synapse or non-synaptic contact. Traditionally, all local transmissions between nerves and effector cells are considered synaptic in nature. This is particularly true for communication between neurons. However, communication between nerves and other effectors such as smooth muscles has been described as nonsynaptic or junctional in nature. Nonsynaptic neurotransmission is now also increasingly recognized in the CNS. This review focuses on the relationship between structure and function that orchestrate synaptic and junctional neurotransmissions. A synapse is a specialized focal contact between the presynaptic active zone capable of ultrafast release of soluble transmitters and the postsynaptic density that cluster ionotropic receptors. The presynaptic and the postsynaptic areas are separated by the 'closed' synaptic cavity. The physiological hallmark of the synapse is ultrafast postsynaptic potentials lasting milliseconds. In contrast, junctions are juxtapositions of nerve terminals and the effector cells without clear synaptic specializations and the junctional space is 'open' to the extracellular space. Based on the nature of the transmitters, postjunctional receptors and their separation from the release sites, the junctions can be divided into 'close' and 'wide' junctions. Functionally, the 'close' and the 'wide' junctions can be distinguished by postjunctional potentials lasting ~1s and tens of seconds, respectively. Both synaptic and junctional communications are common between neurons; however, junctional transmission is the rule at many neuro-non-neural effectors.

Is serotonin in enteric nerves required for distension-evoked peristalsis and propulsion of content in guinea-pig distal colon?

Recent studies have shown genetic deletion of the gene that synthesizes 5-HT in enteric neurons (tryptophan hydroxylase-2, Tph-2) leads to a reduction in intestinal transit. However, deletion of the Tph-2 gene also leads to major developmental changes in enteric ganglia, which could also explain changes in intestinal transit. We sought to investigate this further by acutely depleting serotonin from enteric neurons over a 24-h period, without the confounding influences induced by genetic manipulation. Guinea-pigs were injected with reserpine 24h prior to euthanasia. Video-imaging and spatio-temporal mapping was used to record peristalsis evoked by natural fecal pellets, or slow infusion of intraluminal fluid. Immunohistochemical staining for 5-HT was used to detect the presence of serotonin in the myenteric plexus. It was found that endogenous 5-HT was always detected in myenteric ganglia of control animals, but never in guinea-pigs treated with reserpine. Interestingly, peristalsis was still reliably evoked by either intraluminal fluid, or fecal pellets in reserpine-treated animals that also had their entire mucosa and submucosal plexus removed. In these 5-HT depleted animals, there was no change in the frequency of peristalsis or force generated during peristalsis. In control animals, or reserpine treated animals, high concentrations (up to 10 μM) of ondansetron and SDZ-205-557, or granisetron and SDZ-205-557 had no effect on peristalsis. In summary, acute depletion of serotonin from enteric nerves does not prevent distension-evoked peristalsis, nor propulsion of luminal content. Also, we found no evidence that 5-HT3 and 5-HT4 receptor activation is required for peristalsis, or propulsion of contents to occur. Taken together, we suggest that the intrinsic mechanisms that generate peristalsis and entrain propagation along the isolated guinea-pig distal colon are independent of 5-HT in enteric neurons or the mucosa, and do not require the activation of 5-HT3 or 5-HT4 receptors.

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.

Neurochemical coding compared between varicose axons and cell bodies of myenteric neurons in the guinea-pig ileum

The discrete functional classes of enteric neurons in the mammalian gastrointestinal tract have been successfully distinguished on the basis of the unique combination of molecules and enzymes in their cell bodies ("chemical coding"). Whether the same chemical coding exists in varicose axons of different functional classes has not been systematically tested. In this study, we quantified the coexistence of markers that define classes of nerve cell bodies in the myenteric plexus of the guinea-pig ileum, in varicose axons of the same neurons. Profound differences between the combinations of immunohistochemical markers in myenteric nerve cell bodies and in their varicosities were identified. These discrepancies were particularly notable for classes of neurons that had previously been classified as cholinergic, based on immunoreactivity for choline acetyltransferase (ChAT) in their cell bodies. To detect cholinergic varicose axons of enteric neurons in this study, we used antiserum against the vesicular acetylcholine transporter (VAChT). ChAT-immunoreactivity has been reported to be consistently co-localized with 5-hydroxytryptamine (5-HT) in interneuronal cell bodies, yet only 29±5% (n=4) of 5-HT-immunoreactive varicosities contained vesicular acetylcholine transporter (VAChT). Somatostatin coexists with ChAT-immunoreactivity in a class of descending interneuron but only 21±1% (n=4) of somatostatin-immunoreactive varicosities were VAChT-immunoreactive. Comparable discrepancies were also noted for non-cholinergic markers. The results suggest that chemical coding of cell bodies does not necessarily reflect chemical coding of varicose axon terminals and that the assumption that nerve cell bodies that contain ChAT are functionally cholinergic may be questionable.

Membranes with the same ion channel populations but different excitabilities

Electrical signaling allows communication within and between different tissues and is necessary for the survival of multicellular organisms. The ionic transport that underlies transmembrane currents in cells is mediated by transporters and channels. Fast ionic transport through channels is typically modeled with a conductance-based formulation that describes current in terms of electrical drift without diffusion. In contrast, currents written in terms of drift and diffusion are not as widely used in the literature in spite of being more realistic and capable of displaying experimentally observable phenomena that conductance-based models cannot reproduce (e.g. rectification). The two formulations are mathematically related: conductance-based currents are linear approximations of drift-diffusion currents. However, conductance-based models of membrane potential are not first-order approximations of drift-diffusion models. Bifurcation analysis and numerical simulations show that the two approaches predict qualitatively and quantitatively different behaviors in the dynamics of membrane potential. For instance, two neuronal membrane models with identical populations of ion channels, one written with conductance-based currents, the other with drift-diffusion currents, undergo transitions into and out of repetitive oscillations through different mechanisms and for different levels of stimulation. These differences in excitability are observed in response to excitatory synaptic input, and across different levels of ion channel expression. In general, the electrophysiological profiles of membranes modeled with drift-diffusion and conductance-based models having identical ion channel populations are different, potentially causing the input-output and computational properties of networks constructed with these models to be different as well. The drift-diffusion formulation is thus proposed as a theoretical improvement over conductance-based models that may lead to more accurate predictions and interpretations of experimental data at the single cell and network levels.

Essential roles of enteric neuronal serotonin in gastrointestinal motility and the development/survival of enteric dopaminergic neurons

The gut contains a large 5-HT pool in enterochromaffin (EC) cells and a smaller 5-HT pool in the enteric nervous system (ENS). During development, enteric neurons are generated asynchronously. We tested hypotheses that serotonergic neurons, which arise early, affect development/survival of later-born dopaminergic, GABAergic, nitrergic, and calcitonin gene-related peptide-expressing neurons and are essential for gastrointestinal motility. 5-HT biosynthesis depends on tryptophan hydroxylase 1 (TPH1) in EC cells and on TPH2 in neurons; therefore, mice lacking TPH1 and/or TPH2 distinguish EC-derived from neuronal 5-HT. Deletion of TPH2, but not TPH1, decreased myenteric neuronal density and proportions of dopaminergic and GABAergic neurons but did not affect the extrinsic sympathetic innervation of the gut; intestinal transit slowed in mice lacking TPH2 mice, but gastric emptying accelerated. Isolated enteric crest-derived cells (ENCDCs) expressed the serotonin reuptake transporter (SERT) and 15 subtypes of 5-HT receptor. Addition of 5-HT to cultures of isolated ENCDCs promoted total and dopaminergic neuronal development. Rings of SERT-immunoreactive terminal axons surrounded myenteric dopaminergic neurons and SERT knock-out increased intestinal levels of dopamine metabolites, implying that enteric dopaminergic neurons receive a serotonergic innervation. Observations suggest that constitutive gastrointestinal motility depends more on neuronal than EC cell serotonin; moreover, serotonergic neurons promote development/survival of some classes of late-born enteric neurons, including dopaminergic neurons, which appear to innervate and activate in the adult ENS.

Effects of ischemia and reperfusion on P2X2 receptor expressing neurons of the rat ileum enteric nervous system

PURPOSE

We investigated the effects of ischemia/reperfusion in the intestine (I/R-i) on purine receptor P2X2-immunoreactive (IR) neurons of the rat ileum.

METHODS

The superior mesenteric artery was occluded for 45 min with an atraumatic vascular clamp and animals were sacrificed 4 h later. Neurons of the myenteric and submucosal plexuses were evaluated for immunoreactivity against the P2X2 receptor, nitric oxide synthase (NOS), choline acetyl transferase (ChAT), calbindin, and calretinin.

RESULTS

Following I/R-i, we observed a decrease in P2X2 receptor immunoreactivity in the cytoplasm and surface membranes of neurons of the myenteric and submucosal plexuses. These studies also revealed an absence of calbindin-positive neurons in the I/R-i group. In addition, the colocalization of the P2X2 receptor with NOS, ChAT, and calretinin immunoreactivity in the myenteric plexus was decreased following I/R-i. Likewise, the colocalization between P2X2 and calretinin in neurons of the submucosal plexus was also reduced. In the I/R-i group, there was a 55.8% decrease in the density of neurons immunoreactive (IR) for the P2X2 receptor, a 26.4% reduction in NOS-IR neuron, a 25% reduction in ChAT-IR neuron, and a 47% reduction in calretinin-IR neuron. The density of P2X2 receptor and calretinin-IR neurons also decreased in the submucosal plexus of the I/R-i group. In the myenteric plexus, P2X2-IR, NOS-IR, ChAT-IR and calretinin-IR neurons were reduced in size by 50%, 49.7%, 42%, and 33%, respectively, in the I/R-i group; in the submucosal plexus, P2X2-IR and calretinin-IR neurons were reduced in size by 56% and 72.6%, respectively.

CONCLUSIONS

These data demonstrate that ischemia/reperfusion of the intestine affects the expression of the P2X2 receptor in neurons of the myenteric and submucosal plexus, as well as density and size of neurons in this population. Our findings indicate that I/R-i induces changes in P2X2-IR enteric neurons that could result in alterations in intestinal motility.

Patch clamp recording from enteric neurons in situ

The study of enteric neurons is key to understanding intestinal motility anGutn of therapeutic strategies for dealing with neurogenic disorders. However, enteric neurons have historically been inaccessible to patch-clamp recording. We report here the first technique that allows patch-clamp recording of neurons from the intact myenteric plexus of the mouse duodenum. The mucosa, submucosa and circular muscles are removed, exposing the myenteric plexus on the longitudinal muscle. Proteolytic treatment of exposed ganglia combined with gentle cell-surface cleaning allows gigaseal formation. Compared with previous studies using intracellular microelectrode recordings or cultured myenteric neurons, this technique provides an opportunity to explore properties of single or multiple ion channels in myenteric neurons in their native environment. The protocol-from the tissue preparation to patch-clamp recording-can be completed in ~4 h.