Optical studies of nicotinic acetylcholine receptor subtypes in the guinea-pig enteric nervous system

Indirubin mediates adverse intestinal reactions in guinea pigs by downregulating the expression of AchE through AhR

Indirubin is the main component of the traditional Chinese medicine Indigo naturalis (IN), a potent agonist of aryl hydrocarbon receptors (AhRs). In China, IN is used to treat psoriasis and ulcerative colitis, and indirubin is used for the treatment of chronic myelogenous leukaemia. However, IN and indirubin have adverse reactions, such as abdominal pain, diarrhoea, and intussusception, and their specific mechanism is unclear.The purpose of our research was to determine the specific mechanism underlying the adverse effects of IN and indirubin. By tracking the modifications in guinea pigs after the intragastric administration of indirubin for 28 days.The results demonstrate that indirubin could accelerate bowel movements and decrease intestinal acetylcholinesterase (AchE) expression. Experiments with NCM460 cells revealed that indirubin significantly reduced the expression of AchE, and the AchE levels were increased after the silencing of AhR and re-exposure to indirubin.This study showed that the inhibition of AchE expression by indirubin plays a key role in the occurrence of adverse reactions to indirubin and that the underlying mechanism is related to AhR-mediated AchE downregulation.

Enteric Nervous System: Identification of a Novel Neuronal Sensory Network in the Duodenal Epithelium

The communication between the intestinal epithelium and the enteric nervous system has been considered indirect. Mechanical or chemical stimuli activate enteroendocrine cells inducing hormone secretion, which act on sub-epithelial nerve ends, activating the enteric nervous system. However, we identified an epithelial cell that expresses NKAIN4, a neuronal protein associated with the β-subunit of Na+/K+-ATPase. This cell overexpresses Na+/K+-ATPase and ouabain-insensitive Na+-ATPase, enzymes involved in active sodium transport. NKAIN4-positive cells also express neuronal markers as NeuN, acetylcholine-esterase, acetylcholine-transferase, α3- and α7-subunits of ACh receptors, glutamic-decarboxylase, and serotonin-receptor-7, suggesting they are neurons. NKAIN4-positive cells show a polarized shape with an oval body, an apical process finished in a knob-like terminal in contact with the lumen, a basal cilia body at the base of the apical extension, and basal axon-like soma projections connecting sub-epithelial nerve terminals, lymphoid nodules, glial cells, and enterochromaffin cells, forming a network that reaches the epithelial surface. We also showed, using retrograde labeling and immunofluorescence, that these cells receive afferent signals from the enteric nervous system. Finally, we demonstrated that acetylcholine activates NKAIN4-positive cells inducing Ca2+ mobilization and probably serotonin secretion in enterochromaffin cells. NKAIN4-positive cells are neurons that would form a part of a duodenal sensory network for physiological or noxious luminal stimuli.

Current Practice in Using Voltage Imaging to Record Fast Neuronal Activity: Successful Examples from Invertebrate to Mammalian Studies

The optical imaging of neuronal activity with potentiometric probes has been credited with being able to address key questions in neuroscience via the simultaneous recording of many neurons. This technique, which was pioneered 50 years ago, has allowed researchers to study the dynamics of neural activity, from tiny subthreshold synaptic events in the axon and dendrites at the subcellular level to the fluctuation of field potentials and how they spread across large areas of the brain. Initially, synthetic voltage-sensitive dyes (VSDs) were applied directly to brain tissue via staining, but recent advances in transgenic methods now allow the expression of genetically encoded voltage indicators (GEVIs), specifically in selected neuron types. However, voltage imaging is technically difficult and limited by several methodological constraints that determine its applicability in a given type of experiment. The prevalence of this method is far from being comparable to patch clamp voltage recording or similar routine methods in neuroscience research. There are more than twice as many studies on VSDs as there are on GEVIs. As can be seen from the majority of the papers, most of them are either methodological ones or reviews. However, potentiometric imaging is able to address key questions in neuroscience by recording most or many neurons simultaneously, thus providing unique information that cannot be obtained via other methods. Different types of optical voltage indicators have their advantages and limitations, which we focus on in detail. Here, we summarize the experience of the scientific community in the application of voltage imaging and try to evaluate the contribution of this method to neuroscience research.

TLR2 and TLR4 Modulate Mouse Ileal Motility by the Interaction with Muscarinic and Nicotinic Receptors

Irritable bowel syndrome (IBS) is a chronic functional bowel disorder characterized by intestinal dysmotility. Changes in intestinal microbiota (dysbiosis) can lead to alterations in neuro-muscular functions in the gut. Toll-like receptors (TLRs) 2 and 4 recognize intestinal bacteria and are involved in the motor response induced by gastrointestinal (GI) neurotransmitters. Acetylcholine (ACh) is a well-known neurotransmitter involved in the regulation of GI motility. This study aimed to evaluate the role of TLR2 and TLR4 in the intestinal motor-response induced by ACh in the mouse ileum, as well as the expression and function of the muscarinic and nicotinic ACh receptors. Muscle contractility studies showed that the contractions induced by ACh were significantly lower in TLR2^−/− and TLR4^−/− with respect to WT mice. In WT mice, the contractions induced by ACh were reduced in the presence of AF-DX AF-DX 116 (a muscarinic ACh receptor (mAChR) M2 antagonist), 4-DAMP (a mAChR M3 antagonist), mecamylamine (a nicotinic AChR receptor (nAChR) α3β4 antagonist) and α-bungarotoxin (a nAChR α7 antagonist). In TLR2^−/− mice, the contractions induced by ACh were increased by AF-DX 116 and mecamylamine. In TLR4^−/− mice, the contractions induced by ACh were reduced by α-bungarotoxin and 4-DAMP. The mRNA and protein expressions of M3 and α3 receptors were diminished in the ileum from TLR2^−/− and TLR4^−/− with respect to WT mice. However, the levels of mRNA and protein of β4 were diminished only in TLR4^−/− but not in TLR2^−/− mice. In conclusion, our results show that TLR2 and TLR4 modulates the motor responses to ACh in the mouse ileum. TLR2 acts on muscarinic M2 and M3 and nicotinic α3β4 ACh receptors, while TLR4 acts on muscarinic M3 and nicotinic α3β4 and α7 ACh receptors.

A novel ex vivo model for critical illness neuromyopathy using freshly resected human colon smooth muscle

Patients suffering from critical illness are at risk to develop critical illness neuromyopathy (CINM). The underlying pathophysiology is complex and controversial. A central question is whether soluble serum factors are involved in the pathogenesis of CINM. In this study, smooth muscle preparations obtained from the colon of patients undergoing elective surgery were used to investigate the effects of serum from critically ill patients. At the time of blood draw, CINM was assessed by clinical rating and electrophysiology. Muscle strips were incubated with serum of healthy controls or patients in organ baths and isometric force was measured. Fifteen samples from healthy controls and 98 from patients were studied. Ratios of responses to electric field stimulation (EFS) before and after incubation were 118% for serum from controls and 51% and 62% with serum from critically ill patients obtained at day 3 and 10 of critical illness, respectively (p = 0.003, One-Way-ANOVA). Responses to carbachol and high-K⁺ were equal between these groups. Ratios of post/pre-EFS responses correlated with less severe CINM. These results support the existence of pathogenic, i.e. neurotoxic factors in the serum of critically ill patients. Using human colon smooth muscle as a bioassay may facilitate their future molecular identification.

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.

Nicotinic Acetylcholine Receptor Involvement in Inflammatory Bowel Disease and Interactions with Gut Microbiota

The gut-brain axis describes a complex interplay between the central nervous system and organs of the gastrointestinal tract. Sensory neurons of dorsal root and nodose ganglia, neurons of the autonomic nervous system, and immune cells collect and relay information about the status of the gut to the brain. A critical component in this bi-directional communication system is the vagus nerve which is essential for coordinating the immune system’s response to the activities of commensal bacteria in the gut and to pathogenic strains and their toxins. Local control of gut function is provided by networks of neurons in the enteric nervous system also called the ‘gut-brain’. One element common to all of these gut-brain systems is the expression of nicotinic acetylcholine receptors. These ligand-gated ion channels serve myriad roles in the gut-brain axis including mediating fast synaptic transmission between autonomic pre- and postganglionic neurons, modulation of neurotransmitter release from peripheral sensory and enteric neurons, and modulation of cytokine release from immune cells. Here we review the role of nicotinic receptors in the gut-brain axis with a focus on the interplay of these receptors with the gut microbiome and their involvement in dysregulation of gut function and inflammatory bowel diseases.

Impaired Expression of Neuregulin 1 and Nicotinic Acetylcholine Receptor β4 Subunit in Diverticular Disease

Neuregulin 1 (NRG1) regulates the expression of the nicotinic acetylcholine receptor (nAChR) and is suggested to promote the survival and maintenance of the enteric nervous system (ENS), since deficiency of its corresponding receptor complex ErbB2/ErbB3 leads to postnatal colonic aganglionosis. As diverticular disease (DD) is associated with intestinal hypoganglionosis, the NRG1-ErbB2/ErbB3 system and the nAChR were studied in patients with DD and controls. Samples of tunica muscularis of the sigmoid colon from patients with DD (n = 8) and controls (n = 11) were assessed for mRNA expression of NRG1, ErbB2, and ErbB3 and the nAChR subunits α3, α5, α7, β2, and β4. Site-specific gene expression levels of the NRG1-ErbB2/3 system were determined in myenteric ganglia harvested by laser microdissection (LMD). Localization studies were performed by immunohistochemistry for the NRG1-ErbB2/3 system and nAChR subunit β4. Rat enteric nerve cell cultures were stimulated with NRG1 or glial-cell line derived neurotrophic factor (GDNF) for 6 days and mRNA expression of the aforementioned nAchR was measured. NRG1, ErbB3, and nAChR subunit β4 expression was significantly down-regulated in both the tunica muscularis and myenteric ganglia of patients with DD compared to controls, whereas mRNA expression of ErbB3 and nAChR subunits β2, α3, α5, and α7 remained unaltered. NRG1, ErbB3, and nAChR subunit β4 immunoreactive signals were reduced in neuronal somata and the neuropil of myenteric ganglia from patients with DD compared to control. nAChR subunit β4 exhibited also weaker immunoreactive signals in the tunica muscularis of patients with DD. NRG1 treatment but not GDNF treatment of enteric nerve cell cultures significantly enhanced mRNA expression of nAchR β4. The down-regulation of NRG1 and ErbB3 in myenteric ganglia of patients with DD supports the hypothesis that intestinal hypoganglionosis observed in DD may be attributed to a lack of neurotrophic factors. Regulation of nAChR subunit β4 by NRG1 and decreased nAChR β4 in patients with DD provide evidence that a lack of NRG1 may affect the composition of enteric neurotransmitter receptor subunits thus contributing to the intestinal motility disorders previously reported in DD.

Distribution of α7 Nicotinic Acetylcholine Receptor Subunit mRNA in the Developing Mouse

Homomeric α7 nicotinic acetylcholine receptors (nAChRs) are abundantly expressed in the central and peripheral nervous system (CNS and PNS, respectively), and spinal cord. In addition, expression and functional responses have been reported in non-neuronal tissue. In the nervous system, α7 nAChR subunit expression appears early during embryonic development and is often transiently upregulated, but little is known about their prenatal expression outside of the nervous system. For understanding potential short-term and long-term effects of gestational nicotine exposure, it is important to know the temporal and spatial expression of α7 nAChRs throughout the body. To that end, we studied the expression of α7 nAChR subunit mRNA using highly sensitive isotopic in situ hybridization in embryonic and neonatal whole-body mouse sections starting at gestational day 13. The results revealed expression of α7 mRNA as early as embryonic day 13 in the PNS, including dorsal root ganglia, parasympathetic and sympathetic ganglia, with the strongest expression in the superior cervical ganglion, and low to moderate levels were detected in brain and spinal cord, respectively, which rapidly increased in intensity with embryonic age. In addition, robust α7 mRNA expression was detected in the adrenal medulla, and low to moderate expression in selected peripheral tissues during embryonic development, potentially related to cells derived from the neural crest. Little or no mRNA expression was detected in thymus or spleen, sites of immune cell maturation. The results suggest that prenatal nicotine exposure could potentially affect the nervous system with limited effects in non-neural tissues.

Peripheral N-methyl-D-aspartate receptor localization and role in gastric acid secretion regulation: immunofluorescence and pharmacological studies

The enteric nervous system (ENS) and a glutamate receptor (GluR), N-methyl-D-aspartate receptor (NMDAR), participate in gastric acid secretion (GAS) regulation. NMDARs are localized in different stomach cells; however, knowledge of NMDAR expression and function in the ENS is limited. In the present study, we clarified the types of stomach cells that express the NMDARs that are involved in GAS regulation. The pharmacological method of isolated stomach perfusion by Ghosh and Shild combined with direct mapping of NMDARs by fluorescence microscopy in the rat stomach was employed. By immunofluorescence labeling with an anti-NMDA-NR1 antibody, NMDARs were found to be highly expressed in nerve cells of the submucosal and myenteric plexuses in the stomach. The exact localization of the NMDARs relevant to GAS and its mechanism of action were determined by stimulating different receptors of neuronal and stomach cells using specific secretagogues for NMDA and by selectively blocking those receptors. NMDARs relevant to GAS stimulation are mainly localized in cholinergic interneurons; however, all of the nerve cells of the submucosal ganglia are involved in the stimulating process. In addition, the NMDARs in parietal cells are involved in gastric acid inhibition via influencing H₂-histamine receptors.

Synaptic activation of putative sensory neurons by hexamethonium-sensitive nerve pathways in mouse colon

The gastrointestinal tract contains its own independent population of sensory neurons within the gut wall. These sensory neurons have been referred to as intrinsic primary afferent neurons (IPANs) and can be identified by immunoreactivity to calcitonin gene-related peptide (CGRP) in mice. A common feature of IPANs is a paucity of fast synaptic inputs observed during sharp microelectrode recordings. Whether this is observed using different recording techniques is of particular interest for understanding the physiology of these neurons and neural circuit modeling. Here, we imaged spontaneous and evoked activation of myenteric neurons in isolated whole preparations of mouse colon and correlated recordings with CGRP and nitric oxide synthase (NOS) immunoreactivity, post hoc. Calcium indicator fluo 4 was used for this purpose. Calcium responses were recorded in nerve cell bodies located 5-10 mm oral to transmural electrical nerve stimuli. A total of 618 recorded neurons were classified for CGRP or NOS immunoreactivity. Aboral electrical stimulation evoked short-latency calcium transients in the majority of myenteric neurons, including ~90% of CGRP-immunoreactive Dogiel type II neurons. Activation of Dogiel type II neurons had a time course consistent with fast synaptic transmission and was always abolished by hexamethonium (300 μM) and by low-calcium Krebs solution. The nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium iodide (during synaptic blockade) directly activated Dogiel type II neurons. The present study suggests that murine colonic Dogiel type II neurons receive prominent fast excitatory synaptic inputs from hexamethonium-sensitive neural pathways. NEW & NOTEWORTHY Myenteric neurons in isolated mouse colon were recorded using calcium imaging and then neurochemically defined. Short-latency calcium transients were detected in >90% of calcitonin gene-related peptide-immunoreactive neurons to electrical stimulation of hexamethonium-sensitive pathways. Putative sensory Dogiel type II calcitonin gene-related peptide-immunoreactive myenteric neurons may receive widespread fast synaptic inputs in mouse colon.

Sex Differences and Drug Dose Influence the Role of the α7 Nicotinic Acetylcholine Receptor in the Mouse Dextran Sodium Sulfate-Induced Colitis Model

Introduction

α7 nicotinic acetylcholine receptors (nAChRs) play an important role in vagus nerve-based cholinergic anti-inflammatory effects. This study was designed to assess the role of α7 nAChRs in dextran sodium sulfate (DSS)-induced colitis in male and female mouse. We first compared disease activity and pathogenesis of colitis in α7 knockout and wild-type mice. We then evaluated the effect of several α7 direct and indirect agonists on the severity of disease in the DSS-induced colitis.

Methods

Male and female adult mice were administered 2.5% DSS solution freely in the drinking water for 7 consecutive days and the colitis severity (disease activity index) was evaluated as well as colon length, colon histology, and levels of tumor necrosis factor-alpha colonic levels.

Results

Male, but not female, α7 knockout mice displayed a significantly increased colitis severity and higher tumor necrosis factor-alpha levels as compared with their littermate wild-type mice. Moreover, pretreatment with selective α7 ligands PHA-543613, choline, and PNU-120596 decreased colitis severity in male but not female mice. The anti-colitis effects of these α7 compounds dissipated when administered at higher doses.

Conclusions

Our results suggest the presence of a α7-dependent anti-colitis endogenous tone in male mice. Finally, our results show for the first time that female mice are less sensitive to the anti-colitis activity of α7 agonists. Ovarian hormones may play a key role in the sex difference effect of α7 nAChRs modulation of colitis in the mouse.

Implications

Our collective results suggest that targeting α7 nAChRs could represent a viable therapeutic approach for intestinal inflammation diseases such as ulcerative colitis with the consideration of sex differences.

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.

Reduction of epithelial secretion in male rat distal colonic mucosa by bile acid receptor TGR5 agonist, INT-777: role of submucosal neurons

BACKGROUND

Recent evidence from rat neuron-free mucosa study suggests that the membrane bile acid receptor TGR5 decreases colonic secretion under basal and stimulated conditions. As submucosal neurons are key players in secretory processes and highly express TGR5, we investigated their role in TGR5 agonist-induced inhibition of secretion and the pathways recruited.

METHODS

TGR5 expression and localization were assessed in rat proximal (pC) and distal (dC) colon by qPCR and immunohistochemistry with double labeling for cholinergic neurons in whole-mount preparations. The influence of a selective (INT-777) or weak (ursodeoxycholic acid, UDCA) TGR5 agonist on colonic secretion was assessed in Ussing chambers, in dC preparation removing seromuscular ± submucosal tissues, in the presence of different inhibitors of secretion pathways.

KEY RESULTS

TGR5 mRNA is expressed in full thickness dC and pC and immunoreactivity is located in colonocytes and pChAT-positive neurons. Addition of INT-777, and less potently UDCA, decreased colonic secretion in seromuscular stripped dC by -58.17± 2.6%. INT-777 effect on basal secretion was reduced in neuron-free and TTX-treated mucosal-submucosal preparations. Atropine, hexamethonium, indomethacin, and L-NAME all reduced significantly INT-777's inhibitory effect while the 5-HT₄ antagonist, RS-39604, and lidocaine abolished it. INT-777 inhibited stimulated colonic secretion induced by nicotine, but not cisapride, carbachol or PGE2.

CONCLUSIONS & INFERENCES

TGR5 activation inhibits basal and stimulated distal colonic secretion in rats by acting directly on epithelial cells and also inhibiting submucosal neurons. This could represent a counter-regulatory mechanism, at the submucosal level, of the known prosecretory effect of bile acids in the colon.

Changes in Nicotinic Neurotransmission during Enteric Nervous System Development

Acetylcholine-activating pentameric nicotinic receptors (nAChRs) are an essential mode of neurotransmission in the enteric nervous system (ENS). In this study, we examined the functional development of specific nAChR subtypes in myenteric neurons using Wnt1-Cre;R26R-GCaMP3 mice, where all enteric neurons and glia express the genetically encoded calcium indicator, GCaMP3. Transcripts encoding α3, α4, α7, β2, and β4 nAChR subunits were already expressed at low levels in the E11.5 gut and by E14.5 and, thereafter, α3 and β4 transcripts were the most abundant. The effect of specific nAChR subtype antagonists on evoked calcium activity in enteric neurons was investigated at different ages. Blockade of the α3β4 receptors reduced electrically and chemically evoked calcium responses at E12.5, E14.5, and P0. In addition to the α3β4 antagonist, antagonists to α3β2 and α4β2 also significantly reduced responses by P10-11 and in adult preparations. Therefore, there is an increase in the diversity of functional nAChRs during postnatal development. However, an α7 nAChR antagonist had no effect at any age. Furthermore, at E12.5 we found evidence for unconventional receptors that were responsive to the nAChR agonists 1-dimethyl-4-phenylpiperazinium and nicotine, but were insensitive to the general nicotinic blocker, hexamethonium. Migration, differentiation, and neuritogenesis assays did not reveal a role for nAChRs in these processes during embryonic development. In conclusion, there are significant changes in the contribution of different nAChR subunits to synaptic transmission during ENS development, even after birth. This is the first study to investigate the development of cholinergic transmission in the ENS.

Recent developments in VSD imaging of small neuronal networks

Voltage-sensitive dye (VSD) imaging is a powerful technique that can provide, in single experiments, a large-scale view of network activity unobtainable with traditional sharp electrode recording methods. Here we review recent work using VSDs to study small networks and highlight several results from this approach. Topics covered include circuit mapping, network multifunctionality, the network basis of decision making, and the presence of variably participating neurons in networks. Analytical tools being developed and applied to large-scale VSD imaging data sets are discussed, and the future prospects for this exciting field are considered.

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.

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.

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

Nitric oxide regulation of colonic epithelial ion transport: a novel role for enteric glia in the myenteric plexus

Enteric glia are increasingly recognized as important in the regulation of a variety of gastrointestinal functions.Here we tested the hypothesis that nicotinic signalling in the myenteric plexus results in the release of nitric oxide (NO) from neurons and enteric glia to modulate epithelial ion transport. Ion transport was assessed using full-thickness or muscle-stripped segments of mouse colon mounted in Ussing chambers. The cell-permeant NO-sensitive dye DAR-4M AM and amperometry were utilized to identify the cellular sites of NO production within the myenteric plexus and the contributions from specific NOS isoforms. Nicotinic receptors were localized using immunohistochemistry. Nicotinic cholinergic stimulation of colonic segments resulted in NO-dependent changes in epithelial active electrogenic ion transport that were TTX sensitive and significantly altered in the absence of the myenteric plexus. Nicotinic stimulation of the myenteric plexus resulted in NO production and release from neurons and enteric glia, which was completely blocked in the presence of nitric oxide synthase (NOS) I and NOS II inhibitors. Using the NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO), neuronal and enteric glial components of NO production were demonstrated. Nicotinic receptors were identified on enteric neurons, which express NOS I, and enteric glia, which express NOS II. These data identify a unique pathway in the mouse colon whereby nicotinic cholinergic signalling in myenteric ganglia mobilizes NO from NOS II in enteric glia, which in coordinated activity with neurons in the myenteric plexus modulates epithelial ion transport, a key component of homeostasis and innate immunity.

Validation of independent component analysis for rapid spike sorting of optical recording data

Independent component analysis (ICA) is a technique that can be used to extract the source signals from sets of signal mixtures where the sources themselves are unknown. The analysis of optical recordings of invertebrate neuronal networks with fast voltage-sensitive dyes could benefit greatly from ICA. These experiments can generate hundreds of voltage traces containing both redundant and mixed recordings of action potentials originating from unknown numbers of neurons. ICA can be used as a method for converting such complex data sets into single-neuron traces, but its accuracy for doing so has never been empirically evaluated. Here, we tested the accuracy of ICA for such blind source separation by simultaneously performing sharp electrode intracellular recording and fast voltage-sensitive dye imaging of neurons located in the central ganglia of Tritonia diomedea and Aplysia californica, using a 464-element photodiode array. After running ICA on the optical data sets, we found that in 34 of 34 cases the intracellularly recorded action potentials corresponded 100% to the spiking activity of one of the independent components returned by ICA. We also show that ICA can accurately sort action potentials into single neuron traces from a series of optical data files obtained at different times from the same preparation, allowing one to monitor the network participation of large numbers of individually identifiable neurons over several recording episodes. Our validation of the accuracy of ICA for extracting the neural activity of many individual neurons from noisy, mixed, and redundant optical recording data sets should enable the use of this powerful large-scale imaging approach for studies of invertebrate and suitable vertebrate neuronal networks.

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

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

Optical recording of electrical activity in guinea-pig enteric networks using voltage-sensitive dyes

The enteric nervous system (ENS) is a self-contained network with identified functions, capable of performing complex behaviors in isolation. Its neurons (10 to 25 microm in diameter) are arranged in plexuses that are confined to distinct planes of the gut wall (1); the myenteric plexus can be found between the longitudinal and circular muscle layers, and the submucous plexus between the circular muscle layer and the mucosa. Since the effector systems for these plexuses (transporting epithelium, endocrine cells, immune elements, blood vessels and smooth muscle) are also contained within the gut wall, semi-intact preparations can be dissected that preserve individual components of different reflex pathways. The behavior of the effector systems is controlled by the submucous and myenteric plexuses acting in concert. Therefore, detailed knowledge of synaptic interactions within and between ganglia, and of communication between the plexuses, is essential for understanding normal gastrointestinal function. The ENS, as an intact nervous system, is a unique experimental model in which one can correlate molecular and cellular events with the electrical behavior of the neuronal network and its physiological outputs. Because of the quasi-two-dimensional organization of its plexuses, the ENS is particularly well suited for the study of neural networks using multiple site optical recording techniques that employ voltage-sensitive dyes (2,7,8,9). We will illustrate here the use of a relatively new naphthylstyryl-pyridinium dye (di-4-ANEPPDHQ) (3) that offers multiple advantages over its predecessors, including very low phototoxicity, slow rate of internalization, and remarkable chemical stability. When used in conjunction with a camera that permits sub-millisecond time resolution, this dye allows us to monitor the electrical activity of all the neurons in the field of view with a maximal spatial resolution of approximately 2.5 microm at 100X magnification. At lower magnification (10X or 20X), the sacrifice of single-cell resolution is compensated by a gain in perspective, revealing the intricacies of the inter-ganglionic circuitry.

Specific subtypes of nicotinic cholinergic receptors involved in sympathetic and parasympathetic cardiovascular responses

Various subtypes of nicotinic cholinergic receptors are expressed in autonomic ganglia. The distinct functional roles of these receptors in autonomic ganglionic transmission to different target organs remain to be elucidated. In this study, we tested the sympathetic and parasympathetic cardiovascular responses to nicotinic agonist and antagonists in urethane-anesthetized mice. Intravenous injection with a nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium iodide, induced a brief but pronounced decrease in heart rate, followed by significant increases in heart rate and arterial blood pressure. The bradycardic response was blocked by atropine whereas the pressor response was blocked by prazosine, confirming those responses were parasympathetic and sympathetic activities, respectively. The sympathetic response was blocked by methyllycaconitine citrate, a selective alpha 7 nicotinic cholinergic receptor (nAchR) antagonist. The parasympathetic response was blocked by a selective alpha 4 beta 2 nAchR antagonist, dihydro-beta-erythroidine hydrobromide. Moreover, injection with a selective alpha 4 beta 2 nAchR agonist, RJR2403 oxalate, induced a pronounced parasympathetic response with a smaller sympathetic response. Collectively, these data show that activations of alpha 4 beta 2 nAchRs elicits a parasympathetic cardiovascular response and activation of alpha 7 nAchRs elicits a sympathetic cardiovascular response. These data suggest that specific subtypes of nicotinic receptors at the level of the ganglia may play distinct roles in mediating sympathetic or parasympathetic activation.

Spectral characterization of the voltage-sensitive dye di-4-ANEPPDHQ applied to probing live primary and immortalized neurons

Spectral properties of a recently developed voltage-sensitive dye, di-4-ANEPPDHQ, were characterized as the dye was dissolved in the solvent dimethyl sulfoxide as the stock solution, in Hank's buffered salt solution as the staining solution, and bound to the plasma membrane of primary rat hippocampal neurons and immortalized mouse hypothalamic neurons (GT1-7) in vitro. Their dependence on the local chemical and electrical environment of dye molecules was determined. The excitation and emission peaks are 479 nm and 570 nm for the stained primary neurons, and 476 nm and 585 nm for the stained immortalized neurons. The excitation and emission bands of the stained GT1-7 neurons, defined as 50% peak intensity, are 429-516 nm and 544-648 nm, respectively.

Expression of nicotinic acetylcholine receptors and subunit messenger RNAs in the enteric nervous system of the neonatal rat

In the enteric nervous system (ENS) excitatory nicotinic cholinergic transmission is mediated by neuronal nicotinic acetylcholine receptors (nAChR) and is critical for the regulation of gastric motility. nAChRs are ligand-gated pentameric ion channels found in the CNS and peripheral nervous system. The expression of heteromeric nAChR and receptor subunit mRNAs was investigated in the neonatal rat ENS using receptor autoradiography with the radiolabeled ligand (125)I-epibatidine, and in situ hybridization with subtype specific probes for ligand binding alpha (alpha2, alpha3, alpha4, alpha5, alpha6) and structural beta (beta2, beta3, beta4) subunits. The results showed strong nicotine sensitive binding of (125)I-epibatidine around the stomach, and small and large intestines. The binding was partially displaced by A85380, a nicotinic ligand which differentiates between different heteromeric nAChR subtypes, suggesting a mixed receptor population. Radioactive in situ hybridization detected expression of alpha3, alpha5, alpha7, beta2 and beta4 mRNA in the myenteric plexus of the stomach, and small and large intestines. In the submucosal plexus of the small and large intestines expression of alpha3, alpha5 and beta4 was found in some ganglia. There was no signal for alpha4, alpha6 and beta3 in the ENS but positive hybridization signal for alpha2 transcripts was seen in some areas of the small intestines. However, the signal was not associated with any ganglion cells. The results confirm the presence of heteromeric nAChRs in the ENS similar to those found in the peripheral nervous system, with the majority being composed of alpha3(alpha5)beta4, and a few alpha3beta2 nAChRs. In addition, homomeric alpha7 nAChRs could be present.

Cellular prion protein co-localizes with nAChR beta4 subunit in brain and gastrointestinal tract

PrP(C), the cellular isoform of prion protein, is widely expressed in most tissues, including brain, muscle and gastrointestinal tract. Despite its involvement in several bioprocesses, PrP has still no apparent physiological role. During propagation of transmissible spongiform encephalopathies (TSE), prion protein is converted to the pathological isoform, PrP(Sc), in a process believed to be mediated by unknown host factors. The identification of proteins associated with PrP may provide information about both the biology of prions and the pathogenesis of TSE. Thus far, PrP(C) has been shown to interact with synaptic proteins, components of the cytoskeleton and intracellular proteins involved in signalling pathways. Here, we describe the association of PrP with the beta4 subunit of nicotinic acetylcholine receptor (nAChR), as indicated by co-immunoprecipitation assays and double-label immunofluorescence. The interaction between prion protein and native beta4 subunit was further studied by affinity chromatography, using immobilized and refolded recombinant PrP as a bait and brain homogenates from normal individuals. Additionally, the participation of beta4 subunit in the pathogenesis of TSE was studied by in vivo assays. beta4(-/-) and wild-type mice were challenged with the RML (Rocky Mountain Laboratories) infectious agent. Transgenic animals displayed altered incubation times but the deletion of beta4 subunit did not result in a significant change of the incubation period of the disease. Our results suggest that PrP(C) is a member of a multiprotein membrane complex participating in the formation and function of alpha3beta4 nAChR.

Two-photon excitation of potentiometric probes enables optical recording of action potentials from mammalian nerve terminals in situ

We report the first optical recordings of action potentials, in single trials, from one or a few (approximately 1-2 microm) mammalian nerve terminals in an intact in vitro preparation, the mouse neurohypophysis. The measurements used two-photon excitation along the "blue" edge of the two-photon absorption spectrum of di-3-ANEPPDHQ (a fluorescent voltage-sensitive naphthyl styryl-pyridinium dye), and epifluorescence detection, a configuration that is critical for noninvasive recording of electrical activity from intact brains. Single-trial recordings of action potentials exhibited signal-to-noise ratios of approximately 5:1 and fractional fluorescence changes of up to approximately 10%. This method, by virtue of its optical sectioning capability, deep tissue penetration, and efficient epifluorescence detection, offers clear advantages over linear, as well as other nonlinear optical techniques used to monitor voltage changes in localized neuronal regions, and provides an alternative to invasive electrode arrays for studying neuronal systems in vivo.

Functional interactions between the SK2 channel and the nicotinic acetylcholine receptor in enteric neurons of the guinea pig ileum

The neurotransmitter acetylcholine (ACh) plays a critical role in gastrointestinal function. The role of the small conductance Ca2+-activated K+ (SK) channel in ACh release was examined using myenteric plexus preparations of guinea pig ileum. Apamin, an inhibitor of the SK channel, significantly enhanced nicotine-induced ACh release, but neither electrical field stimulation- nor 5-hydroxytryptamine-induced ACh release, suggesting that SK channels might be selectively involved in the regulation of nicotine-induced ACh release. Therefore, we investigated the distribution of SK2 and SK3 subunits and the interaction between SK2 channels and nicotinic ACh receptors (nAChRs) in the guinea pig ileum. The immunoreactivity of SK2 subunits was located in enteric neuronal cells. Furthermore, SK2-immunoreactive cells stained with an antibody for choline acetyltransferase, a marker for cholinergic neurons, and with an antibody for the alpha3/5 subunits of nAChR. In contrast, immunoreactivity of SK3 subunits was not found in enteric neurons. A co-immunoprecipitation assay with Triton X-100-soluble membrane fractions prepared from the ileum revealed an association of the SK2 subunit with the alpha3/5 subunits of nAChR. These results suggest that SK2 channels negatively regulate the excitation of enteric neurons via functional interactions with nAChRs.

Enteric nervous system

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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