Correlation of electrophysiological and morphological characteristics of enteric neurons in the mouse colon

Enteric glutamatergic interneurons regulate intestinal motility

The enteric nervous system (ENS) controls digestion autonomously via a complex neural network within the gut wall. Enteric neurons expressing glutamate have been identified by transcriptomic studies as a distinct subpopulation, and glutamate can affect intestinal motility by modulating enteric neuron activity. However, the nature of glutamatergic neurons, their position within the ENS circuit, and their function in regulating gut motility are unknown. We identify glutamatergic neurons as longitudinally projecting descending interneurons in the small intestine and colon and as a novel class of circumferential neurons only in the colon. Both populations make synaptic contact with diverse neuronal subtypes and signal with multiple neurotransmitters and neuropeptides in addition to glutamate, including acetylcholine and enkephalin. Knocking out the glutamate transporter VGLUT2 from enkephalin neurons disrupts gastrointestinal transit, while ex vivo optogenetic stimulation of glutamatergic neurons initiates colonic propulsive motility. Our results posit glutamatergic neurons as key interneurons that regulate intestinal motility.

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

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

Synaptic cell adhesion molecule Cdh6 identifies a class of sensory neurons with novel functions in colonic motility

Intrinsic sensory neurons are an essential part of the enteric nervous system (ENS) and play a crucial role in gastrointestinal tract motility and digestion. Neuronal subtypes in the ENS have been distinguished by their electrophysiological properties, morphology, and expression of characteristic markers, notably neurotransmitters and neuropeptides. Here we investigated synaptic cell adhesion molecules as novel cell type markers in the ENS. Our work identifies two Type II classic cadherins, Cdh6 and Cdh8, specific to sensory neurons in the mouse colon. We show that Cdh6+ neurons demonstrate all other distinguishing classifications of enteric sensory neurons including marker expression of Calcb and Nmu, Dogiel type II morphology and AH-type electrophysiology and I H current. Optogenetic activation of Cdh6+ sensory neurons in distal colon evokes retrograde colonic motor complexes (CMCs), while pharmacologic blockade of rhythmicity-associated current I H disrupts the spontaneous generation of CMCs. These findings provide the first demonstration of selective activation of a single neurochemical and functional class of enteric neurons, and demonstrate a functional and critical role for sensory neurons in the generation of CMCs.

Morphological, molecular, and functional characterization of mouse glutamatergic myenteric neurons

The enteric nervous system (ENS) functions largely independently of the central nervous system (CNS). Glutamate, the dominant neurotransmitter in the CNS and sensory afferents, is not a primary neurotransmitter in the ENS. Only a fraction (∼2%) of myenteric neurons in the mouse distal colon and rectum (colorectum) are positive for vesicular glutamate transporter type 2 (VGLUT2), the structure and function of which remain undetermined. Here, we systematically characterized VGLUT2-positive enteric neurons (VGLUT2-ENs) through sparse labeling with adeno-associated virus, single-cell mRNA sequencing (scRNA-seq), and GCaMP6f calcium imaging. Our results reveal that the majority of VGLUT2-ENs (29 of 31, 93.5%) exhibited Dogiel type I morphology with a single aborally projecting axon; most axons (26 of 29, 89.7%) are between 4 and 10 mm long, each traversing 19 to 34 myenteric ganglia. These anatomical features exclude the VGLUT2-ENs from being intrinsic primary afferent or motor neurons. The scRNA-seq conducted on 52 VGLUT2-ENs suggests different expression profiles from conventional descending interneurons. Ex vivo GCaMP6f recordings from flattened colorectum indicate that almost all VGLUT2-EN (181 of 215, 84.2%) are indirectly activated by colorectal stretch via nicotinic cholinergic neural transmission. In conclusion, VGLUT2-ENs are a functionally unique group of enteric neurons with single aborally projecting long axons that traverse multiple myenteric ganglia and are activated indirectly by colorectal mechanical stretch. This knowledge will provide a solid foundation for subsequent studies on the potential interactions of VGLUT2-EN with extrinsic colorectal afferents via glutamatergic neurotransmission.NEW & NOTEWORTHY We reveal that VGLUT2-positive enteric neurons (EN), although constituting a small fraction of total EN, are homogeneously expressed in the myenteric ganglia, with a slight concentration at the intermediate region between the colon and rectum. Through anatomic, molecular, and functional analyses, we demonstrated that VGLUT2-ENs are activated indirectly by noxious circumferential colorectal stretch via nicotinic cholinergic transmission, suggesting their participation in mechanical visceral nociception.

Comparison of wholemount dissection methods for neuronal subtype marker expression in the mouse myenteric plexus

BACKGROUND

Accurately reporting the identity and representation of enteric nervous system (ENS) neuronal subtypes along the length of the gastrointestinal (GI) tract is critical to advancing our understanding of ENS control of GI function. Reports of varying proportions of subtype marker expression have employed different dissection techniques to achieve wholemount muscularis preparations of myenteric plexus. In this study, we asked whether differences in GI dissection methods could introduce variability into the quantification of marker expression.

METHODS

We compared three commonly used methods of ENS wholemount dissection: two flat-sheet preparations that differed in the order of microdissection and fixation and a third rod-mounted peeling technique. We also tested a reversed orientation variation of flat-sheet peeling, two step-by-step variations of the rod peeling technique, and whole-gut fixation as a tube. We assessed marker expression using immunohistochemistry, genetic reporter lines, confocal microscopy, and automated image analysis.

KEY RESULTS AND CONCLUSIONS

We found no significant differences between the two flat-sheet preparation methods in the expression of calretinin or neuronal nitric oxide synthase (nNOS) as a proportion of total neurons in ileum myenteric plexus. However, the rod-mounted peeling method resulted in decreased proportion of neurons labeled for both calretinin and nNOS. This method also resulted in decreased transgenic reporter fluorescent protein (tdTomato) for substance P in distal colon and choline acetyltransferase (ChAT) in both ileum and distal colon. These results suggest that labeling among some markers, both native protein and transgenic fluorescent reporters, is decreased by the rod-mounted mechanical method of peeling. The step-by-step variations of this method point to mechanical manipulation of the tissue as the likely cause of decreased labeling. Our study thereby demonstrates a critical variability in wholemount muscularis dissection methods.

Morphological, molecular, and functional characterization of mouse glutamatergic myenteric neurons

The enteric nervous system (ENS) functions largely independently of the central nervous system (CNS). Correspondingly, glutamate, the dominant neurotransmitter in the CNS and sensory afferents, is not a primary neurotransmitter in the ENS. Only a fraction (approximately 2%) of myenteric neurons in the mouse distal colon and rectum (colorectum) are positive for vesicular glutamate transporter type 2 (VGLUT2), the structure and function of which remain undetermined. Here, we systematically characterized VGLUT2-positive enteric neurons (VGLUT2-ENs) through sparse labeling with adeno-associated virus, single-cell mRNA sequencing (scRNA-seq), and GCaMP6f calcium imaging. Our results reveal that the majority of VGLUT2-ENs (29 out of 31, 93.5%) exhibited Dogiel type I morphology with a single aborally projecting axon; most axons (26 out of 29, 89.7%) are between 4 and 10 mm long, each traversing 19 to 34 myenteric ganglia. These anatomical features exclude the VGLUT2-ENs from being intrinsic primary afferent or motor neurons. The scRNA-seq conducted on 52 VGLUT2-ENs suggests different expression profiles from conventional descending interneurons. Ex vivo GCaMP6f recordings from flattened colorectum indicate that almost all VGLUT2-EN (181 out of 215, 84.2%) are indirectly activated by colorectal stretch via nicotinic cholinergic neural transmission. In conclusion, VGLUT2-ENs are a functionally unique group of enteric neurons with single aborally projecting long axons that traverse multiple myenteric ganglia and are activated indirectly by colorectal mechanical stretch. This knowledge will provide a solid foundation for subsequent studies on the potential interactions of VGLUT2-EN with extrinsic colorectal afferents via glutamatergic neurotransmission.

New & Noteworthy

We reveal that VGLUT2-positive enteric neurons (EN), although constituting a small fraction of total EN, are homogeneously expressed in the myenteric ganglia, with a slight concentration at the intermediate region between the colon and rectum. This concentration coincides with the entry zone of extrinsic afferents into the colorectum. Given that VGLUT2-ENs are indirectly activated by colorectal mechanical stretch, they are likely to participate in visceral nociception through glutamatergic neural transmission with extrinsic afferents.

The autonomic nervous system from a morphofunctional perspective: Historical overview and current concepts over the last two centuries highlighting contributions from Eastern Europe

The present contribution comprises both an introductory comment and an overview of the contributions within this special issue on historical and current research on the autonomic nervous system from Eastern European colleagues, particularly focusing on the autonomic innervation of the gastrointestinal tract and of the cardiovascular system. It also gives a selected overview of interesting and seminal papers on these topics that appeared in The Anatomical Record since its foundation in 1906.

Quantitative Spatial Analysis of Neuroligin-3 mRNA Expression in the Enteric Nervous System Reveals a Potential Role in Neuronal-Glial Synapses and Reduced Expression in Nlgn3R451C Mice

Mutations in the Neuroligin-3 (Nlgn3) gene are implicated in autism spectrum disorder (ASD) and gastrointestinal (GI) dysfunction, but cellular Nlgn3 expression in the enteric nervous system remains to be characterised. We combined RNAScope in situ hybridization and immunofluorescence to measure Nlgn3 mRNA expression in cholinergic and VIP-expressing submucosal neurons, nitrergic and calretinin-containing myenteric neurons and glial cells in both WT and Nlgn3^R451C mutant mice. We measured Nlgn3 mRNA neuronal and glial expression via quantitative three-dimensional image analysis. To validate dual RNAScope/immunofluorescence data, we interrogated available single-cell RNA sequencing (scRNASeq) data to assess for Nlgn3, Nlgn1, Nlgn2 and their binding partners, Nrxn1-3, MGDA1 and MGDA2, in enteric neural subsets. Most submucosal and myenteric neurons expressed Nlgn3 mRNA. In contrast to other Nlgns and binding partners, Nlgn3 was strongly expressed in enteric glia, suggesting a role for neuroligin-3 in mediating enteric neuron-glia interactions. The autism-associated R451C mutation reduces Nlgn3 mRNA expression in cholinergic but not in VIPergic submucosal neurons. In the myenteric plexus, Nlgn3 mRNA levels are reduced in calretinin, nNOS-labelled neurons and S100 β -labelled glia. We provide a comprehensive cellular profile for neuroligin-3 expression in ileal neuronal subpopulations of mice expressing the R451C autism-associated mutation in Nlgn3, which may contribute to the understanding of the pathophysiology of GI dysfunction in ASD.

The enteric nervous system

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

Electrophysiological and morphological features of myenteric neurons of human colon revealed by intracellular recording and dye fills

BACKGROUND

Ex vivo intracellular recordings and dye fills, combined with immunohistochemistry, are a powerful way to analyze the enteric nervous system of laboratory animals.

METHODS

Myenteric neurons were recorded in isolated specimens of human colon. A key determinant of successful recording was near-complete removal of circular muscle from the surface of ganglia.

KEY RESULTS

Treatment with a collagenase/neutral protease mix before dissection significantly improved recording success and reduced damage to the plexus. Carboxyfluorescein in microelectrodes allowed recorded neurons to be routinely labeled, analyzed, and subjected to multi-layer immunohistochemistry. Carboxyfluorescein revealed morphological details that were not detected by immunohistochemical methods. Of 54 dye-filled myenteric neurons (n = 22), 45 were uni-axonal and eight were multi-axonal. There was a significant bias toward recordings from large neural somata. The close association between morphology and electrophysiology (long after-hyperpolarizations and fast EPSPs) seen in mice and guinea pigs did not hold for human myenteric neuron recordings. No slow EPSPs were recorded; however, disruption to the myenteric plexus during dissection may have led the proportion of cells receiving synaptic potentials to be underestimated. Neurons immunoreactive for nitric oxide synthase were more excitable than non-immunoreactive neurons. Distinctive grooves were observed on the serosal and/or mucosal faces of myenteric neurons in 3D reconstructions. These had varicose axons running through them and may represent a preferential site of synaptic inputs.

CONCLUSIONS

Human enteric neurons share many features with laboratory animals, but the combinations of features in individual cells appear more variable.

Comparison of wholemount dissection methods for neuronal subtype marker expression in the mouse myenteric plexus

Background

Accurately reporting the identity and representation of enteric nervous system (ENS) neuronal subtypes along the length of the gastrointestinal (GI) tract is critical to advancing our understanding of ENS control of GI tract function. Reports of varying proportions of subtype marker expression have employed different dissection techniques to achieve wholemount muscularis preparations of myenteric plexus. In this study we asked whether differences in GI dissection methods could introduce variability into the quantification of marker expression.

Methods

We compared three commonly used methods of ENS wholemount dissection: two flat-sheet preparations that differed in the order of microdissection and fixation as well as a rod-mounted peeling technique. We assessed marker expression using immunohistochemistry, genetic reporter lines, confocal microscopy, and automated image analysis.

Key Results and Conclusions

We found no significant differences between the two flat-sheet preparation methods in the expression of calretinin, neuronal nitric oxide synthase (nNOS), or somatostatin (SST) in ileum myenteric plexus. However, the rod-mounted peeling method resulted in decreased marker labeling for both calretinin and nNOS. This method also resulted in decreased transgenic reporter fluorescent protein (tdTomato) for substance P in ileum and choline acetyltransferase (ChAT) in both ileum and distal colon. These results suggest that labeling among some markers, both native protein and transgenic fluorescent reporters, is decreased by the rod-mounted mechanical method of peeling, demonstrating a critical variability in wholemount muscularis dissection methods.

A functional network of highly pure enteric neurons in a dish

The enteric nervous system (ENS) is the intrinsic nervous system that innervates the entire digestive tract and regulates major digestive functions. Recent evidence has shown that functions of the ENS critically rely on enteric neuronal connectivity; however, experimental models to decipher the underlying mechanisms are limited. Compared to the central nervous system, for which pure neuronal cultures have been developed for decades and are recognized as a reference in the field of neuroscience, an equivalent model for enteric neurons is lacking. In this study, we developed a novel model of highly pure rat embryonic enteric neurons with dense and functional synaptic networks. The methodology is simple and relatively fast. We characterized enteric neurons using immunohistochemical, morphological, and electrophysiological approaches. In particular, we demonstrated the applicability of this culture model to multi-electrode array technology as a new approach for monitoring enteric neuronal network activity. This in vitro model of highly pure enteric neurons represents a valuable new tool for better understanding the mechanisms involved in the establishment and maintenance of enteric neuron synaptic connectivity and functional networks.

Single Nucleus Sequencing of Human Colon Myenteric Plexus-Associated Visceral Smooth Muscle Cells, Platelet Derived Growth Factor Receptor Alpha Cells, and Interstitial Cells of Cajal

BACKGROUND AND AIMS

Smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and platelet-derived growth factor receptor alpha (PDGFRα+) cells (PαCs) form a functional syncytium in the bowel known as the "SIP syncytium." The SIP syncytium works in concert with the enteric nervous system (ENS) to coordinate bowel motility. However, our understanding of individual cell types that form this syncytium and how they interact with each other remains limited, with no prior single-cell RNAseq analyses focused on human SIP syncytium cells.

METHODS

We analyzed single-nucleus RNA sequencing data from 10,749 human colon SIP syncytium cells (5572 SMC, 372 ICC, and 4805 PαC nuclei) derived from 15 individuals.

RESULTS

Consistent with critical contractile and pacemaker functions and with known enteric nervous system interactions, SIP syncytium cell types express many ion channels, including mechanosensitive channels in ICCs and PαCs. PαCs also prominently express extracellular matrix-associated genes and the inhibitory neurotransmitter receptor for vasoactive intestinal peptide (VIPR2), a novel finding. We identified 2 PαC clusters that differ in the expression of many ion channels and transcriptional regulators. Interestingly, SIP syncytium cells co-express 6 transcription factors (FOS, MEIS1, MEIS2, PBX1, SCMH1, and ZBTB16) that may be part of a combinatorial signature that specifies these cells. Bowel region-specific differences in SIP syncytium gene expression may correlate with regional differences in function, with right (ascending) colon SMCs and PαCs expressing more transcriptional regulators and ion channels than SMCs and PαCs in left (sigmoid) colon.

CONCLUSION

These studies provide new insights into SIP syncytium biology that may be valuable for understanding bowel motility disorders and lead to future investigation of highlighted genes and pathways.

Environmental perception and control of gastrointestinal immunity by the enteric nervous system

The enteric nervous system (ENS) forms a versatile sensory system along the gastrointestinal tract that interacts with most cell types in the bowel. Herein, we portray host-environment interactions at the intestinal mucosal surface through the lens of the enteric nervous system. We describe local cellular interactions as well as long-range circuits between the enteric, central, and peripheral nervous systems. Additionally, we discuss recently discovered mechanisms by which enteric neurons and glia respond to biotic and abiotic environmental changes and how they regulate intestinal immunity and inflammation. The enteric nervous system emerges as an integrative sensory system with manifold immunoregulatory functions under both homeostatic and pathophysiological conditions.

Simultaneous whole-cell patch-clamp and calcium imaging on myenteric neurons

Live calcium imaging is often used as a proxy for electrophysiological measurements and has been a valuable tool that allows simultaneous analysis of neuronal activity in multiple cells at the population level. In the enteric nervous system, there are two main electrophysiological classes of neurons, after-hyperpolarizing (AH)- and synaptic (S)-neurons, which have been shown to have different calcium handling mechanisms. However, they are rarely considered separately in calcium imaging experiments. A handful of studies have shown that in guinea pig, a calcium transient will accompany a single action potential in AH-neurons, but multiple action potentials are required to generate a calcium transient in S-neurons. How this translates to different modes of cellular depolarization and whether this is consistent across species is unknown. In this study, we used simultaneous whole-cell patch-clamp electrophysiology together with calcium imaging to investigate how enteric neurons respond to different modes of depolarization. Using both traditional (4 Hz) and also high-speed (1,000 Hz) imaging techniques, we found that single action potentials elicit calcium transients in both AH-neurons and S-neurons. Subthreshold membrane depolarizations were also able to elicit calcium transients, although calcium responses were generally amplified if an action potential was present. Furthermore, we identified that responses to nicotinic acetylcholine receptor stimulation can be used to distinguish between AH- and S-neurons in calcium imaging.NEW & NOTEWORTHY Live calcium imaging is an important tool for investigating enteric nervous system (ENS) function. Previous studies have shown that multiple action potentials are needed to generate a calcium response in S-neurons, which has important implications for the interpretation of calcium imaging data. Here, we show that in mouse myenteric neurons, calcium transients are elicited by single action potentials in both AH- and S-neurons. In addition, nicotinic acetylcholine receptor stimulation can be used to distinguish between these two classes.

Quantification of CGRP-immunoreactive myenteric neurons in mouse colon

Quantitative data of biological systems provide valuable baseline information for understanding pathology, experimental perturbations, and computational modeling. In mouse colon, calcitonin gene-related peptide (CGRP) is expressed by myenteric neurons with multiaxonal (Dogiel type II) morphology, characteristic of intrinsic primary afferent neurons (IPANs). Analogous neurons in other species and gut regions represent 5-35% of myenteric neurons. We aimed to quantify proportions of CGRP-immunopositive (CGRP+) myenteric neurons. Colchicine-treated wholemount preparations of proximal, mid, and distal colon were labeled for HuC/D, CGRP, nitric oxide synthase (NOS), and peripherin (Per). The pan-neuronal markers (Hu+/Per+) co-labeled 94% of neurons. Hu+/Per- neurons comprised ∼6%, but Hu-/Per+ cells were rare. Thus, quantification was based on Hu+ myenteric neurons (8576 total; 1225 ± 239 per animal, n = 7). CGRP+ cell bodies were significantly larger than the average of all Hu+ neurons (329 ± 13 vs. 261 ± 12 μm² , p < .0001). CGRP+ neurons comprised 19% ± 3% of myenteric neurons without significant regional variation. NOS+ neurons comprised 42% ± 2% of myenteric neurons overall, representing a lower proportion in proximal colon, compared to mid and distal colon (38% ± 2%, 44% ± 2%, and 44% ± 3%, respectively). Peripherin immunolabeling revealed cell body and axonal morphology in some myenteric neurons. Whether all CGRP+ neurons were multiaxonal could not be addressed using peripherin immunolabeling. However, of 118 putatively multiaxonal neurons first identified based on peripherin immunoreactivity, all were CGRP+ (n = 4). In conclusion, CGRP+ myenteric neurons in mouse colon were comprehensively quantified, occurring within a range expected of a putative IPAN marker. All Per+ multiaxonal neurons, characteristic of Dogiel type II/IPAN morphology, were CGRP+.

Present and future avenues of cell-based therapy for brain injury: The enteric nervous system as a potential cell source

Cell therapy is a promising strategy in the field of regenerative medicine; however, several concerns limit the effective clinical use, namely a valid cell source. The gastrointestinal tract, which contains a highly organized network of nerves called the enteric nervous system (ENS), is a valuable reservoir of nerve cells. Together with neurons and neuronal precursor cells, it contains glial cells with a well described neurotrophic potential and a newly identified neurogenic one. Recently, enteric glia is looked at as a candidate for cell therapy in intestinal neuropathies. Here, we present the therapeutic potential of the ENS as cell source for brain repair, too. The example of stroke is introduced as a brain injury where cell therapy appears promising. This disease is the first cause of handicap in adults. The therapies developed in recent years allow a partial response to the consequences of the disease. The only prospect of recovery in the chronic phase is currently based on rehabilitation. The urgency to offer other treatments is therefore tangible. In the first part of the review, some elements of stroke pathophysiology are presented. An update on the available therapeutic strategies is provided, focusing on cell‐ and biomaterial‐based approaches. Following, the ENS is presented with its anatomical and functional characteristics, focusing on glial cells. The properties of these cells are depicted, with particular attention to their neurotrophic and, recently identified, neurogenic properties. Finally, preliminary data on a possible therapeutic approach combining ENS‐derived cells and a biomaterial are presented.

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.

Rhythmicity in the Enteric Nervous System of Mice

The enteric nervous system (ENS) is required for many cyclical patterns of motor activity along different regions of the gastrointestinal (GI) tract. What has remained mysterious is precisely how many thousands of neurons within the ENS are temporally activated to generate cyclical neurogenic contractions of GI-smooth muscle layers. This has been an especially puzzling conundrum, since the ENS consists of an extensive network of small ganglia, with each ganglion consisting of a heterogeneous population of neurons, with diverse cell soma morphologies, neurochemical and biophysical characteristics, and neural connectivity. Neuronal imaging studies of the mouse large intestine have provided major new insights into how the different classes of myenteric neurons are activated during cyclical neurogenic motor patterns, such as the colonic motor complex (CMC). It has been revealed that during CMCs (in the isolated mouse whole colon), large populations of myenteric neurons, across large spatial fields, coordinate their firing, via bursts of fast synaptic inputs at ~2 Hz. This coordinated firing of many thousands of myenteric neurons synchronously over many rows of interconnected ganglia occurs irrespective of the functional class of neuron. Aborally directed propulsion of content along the mouse colon is due, in large part, to polarity of the enteric circuits including the projections of the intrinsic excitatory and inhibitory motor neurons but still involves the fundamental ~2 Hz rhythmic activity of specific classes of enteric neurons. What remains to be determined are the mechanisms that initiate and terminate the patterned firing of large ensembles of enteric neurons during cyclic activity. This remains an exciting challenge for future studies.

Refining Enteric Neural Circuitry by Quantitative Morphology and Function in Mice

RNA-Seq, electrophysiology and optogenetics in mouse models are used to assess function, identify disease related genes and model enteric neural circuits. Lacking a comprehensive quantitative description of the murine colonic enteric nervous system (ENS) makes it difficult to most effectively use mouse data to better understand ENS function or for development of therapeutic approaches for human motility disorders. Our goal was to provide a quantitative description of mouse colon to establish the extent to which mouse colon architecture, connectivity and function is a useful surrogate for human and other mammalian ENS. Using GCaMP imaging coupled with pharmacology and quantitative confocal and 3D image reconstruction, we present quantitative and functional data demonstrating that regional structural changes and variable distribution of neurons define neural circuit dynamics and functional connectivity responsible for colonic motor patterns and regional functional differences. Our results advance utility of multispecies and gut region-specific data.

Activation of ENS Circuits in Mouse Colon: Coordination in the Mouse Colonic Motor Complex as a Robust, Distributed Control System

The characteristic motor patterns of the colon are coordinated by the enteric nervous system (ENS) and involve enterochromaffin (EC) cells, enteric glia, smooth muscle fibers, and interstitial cells. While the fundamental control mechanisms of colonic motor patterns are understood, greater complexity in the circuitry underlying motor patterns has been revealed by recent advances in the field. We review these recent advances and new findings from our laboratories that provide insights into how the ENS coordinates motor patterns in the isolated mouse colon. We contextualize these observations by describing the neuromuscular system underling the colonic motor complex (CMC) as a robust, distributed control system. Framing the colonic motor complex as a control system reveals a new perspective on the coordinated motor patterns in the colon. We test the control system by applying electrical stimulation in the isolated mouse colon to disrupt the coordination and propagation of the colonic motor complex.

Optogenetic activation of the distal colon epithelium engages enteric nervous system circuits to initiate motility patterns

Digestive functions of the colon depend on sensory-motor reflexes in the enteric nervous system (ENS), initiated by intrinsic primary afferent neurons (IPANs). IPAN terminals project to the mucosal layer of the colon, allowing communication with epithelial cells comprising the colon lining. The chemical nature and functional significance of this epithelial-neural communication in regard to secretion and colon motility are of high interest. Colon epithelial cells can produce and release neuroactive substances such as ATP and 5-hydroxytryptamine (5-HT), which can activate receptors on adjacent nerve fibers, including IPAN subtypes. In this study, we examined if stimulation of epithelial cells alone is sufficient to activate neural circuits that control colon motility. Optogenetics and calcium imaging were used in ex vivo preparations of the mouse colon to selectively stimulate the colon epithelium, measure changes in motility, and record activity of neurons within the myenteric plexus. Light-mediated activation of epithelial cells lining the distal, but not proximal, colon caused local contractions and increased the rate of colonic migrating motor complexes. Epithelial-evoked local contractions in the distal colon were reduced by both ATP and 5-HT receptor antagonists. Our findings indicate that colon epithelial cells likely use purinergic and serotonergic signaling to initiate activity in myenteric neurons, produce local contractions, and facilitate large-scale coordination of ENS activity responsible for whole colon motility patterns.NEW & NOTEWORTHY Using an all-optical approach to measure real-time cell-to-cell communication responsible for colon functions, we show that selective optogenetic stimulation of distal colon epithelium produced activity in myenteric neurons, as measured with red genetically encoded calcium indicators. The epithelial-induced neural response led to local contractions, mediated by both purinergic and serotonergic signaling, and facilitated colonic motor complexes that propagate from proximal to distal colon.

Unique Neural Circuit Connectivity of Mouse Proximal, Middle, and Distal Colon Defines Regional Colonic Motor Patterns

BACKGROUND & AIMS

Colonic motor patterns have been described by a number of different groups, but the neural connectivity and ganglion architecture supporting patterned motor activity have not been elucidated. Our goals were to describe quantitatively, by region, the structural architecture of the mouse enteric nervous system and use functional calcium imaging, pharmacology, and electrical stimulation to show regional underpinnings of different motor patterns.

METHODS

Excised colon segments from mice expressing the calcium indicator GCaMP6f or GCaMP6s were used to examine spontaneous and evoked (pharmacologic or electrical) changes in GCaMP-mediated fluorescence and coupled with assessment of colonic motor activity, immunohistochemistry, and confocal imaging. Three-dimensional image reconstruction and statistical methods were used to describe quantitatively mouse colon myenteric ganglion structure, neural and vascular network patterning, and neural connectivity.

RESULTS

In intact colon, regionally specific myenteric ganglion size, architecture, and neural circuit connectivity patterns along with neurotransmitter-receptor expression underlie colonic motor patterns that define functional differences along the colon. Region-specific effects on spontaneous, evoked, and chemically induced neural activity contribute to regional motor patterns, as does intraganglionic functional connectivity. We provide direct evidence of neural circuit structural and functional regional differences that have only been inferred in previous investigations. We include regional comparisons between quantitative measures in mouse and human colon that represent an important advance in showing the usefulness and relevance of the mouse system for translation to the human colon.

CONCLUSIONS

There are several neural mechanisms dependent on myenteric ganglion architecture and functional connectivity that underlie neurogenic control of patterned motor function in the mouse colon.

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.

Microbiota-gut-brain axis: enteroendocrine cells and the enteric nervous system form an interface between the microbiota and the central nervous system

The microbiota-gut-brain axis transmits bidirectional communication between the gut and the central nervous system and links the emotional and cognitive centers of the brain with peripheral gut functions. This communication occurs along the axis via local, paracrine, and endocrine mechanisms involving a variety of gut-derived peptide/amine produced by enteroendocrine cells. Neural networks, such as the enteric nervous system, and the central nervous system, including the autonomic nervous system, also transmit information through the microbiota-gut-brain axis. Recent advances in research have described the importance of the gut microbiota in influencing normal physiology and contributing to disease. We are only beginning to understand this bidirectional communication system. In this review, we summarize the available data supporting the existence of these interactions, highlighting data related to the contribution of enteroendocrine cells and the enteric nervous system as an interface between the gut microbiota and brain.

Multicolor sparse viral labeling and 3D digital tracing of enteric plexus in mouse proximal colon using a novel adeno-associated virus capsid

BACKGROUND

Intravenous administration of adeno-associated virus (AAV) can be used as a noninvasive approach to trace neuronal morphology and links. AAV-PHP.S is a variant of AAV9 that effectively transduces the peripheral nervous system. The objective was to label randomly and sparsely enteric plexus in the mouse colon using AAV-PHP.S with a tunable two-component multicolor vector system and digitally trace individual neurons and nerve fibers within microcircuits in three dimensions (3D).

METHODS

A vector system including a tetracycline inducer with a tet-responsive element driving three separate fluorophores was packaged in the AAV-PHP.S capsid. The vectors were injected retro-orbitally in mice, and the colon was harvested 3 weeks after. Confocal microscopic images of enteric plexus were digitally segmented and traced in 3D using Neurolucida 360, neuTube, or Imaris software.

KEY RESULTS

The transduction of multicolor AAV vectors induced random sparse spectral labeling of soma and neurites primarily in the myenteric plexus of the proximal colon, while neurons in the submucosal plexus were occasionally transduced. Digital tracing in 3D showed various types of wiring, including multiple conjunctions of one neuron with other neurons, neurites en route, and endings; clusters of neurons in close apposition between each other; axon-axon parallel conjunctions; and intraganglionic nerve endings consisting of multiple nerve endings and passing fibers. Most of digitally traced neuronal somas were of small or medium in size.

CONCLUSIONS & INFERENCES

The multicolor AAV-PHP.S-packaged vectors enabled random sparse spectral labeling and revealed complexities of enteric microcircuit in the mouse proximal colon. The techniques can facilitate digital modeling of enteric micro-circuitry.

Classification of human enteric neurons

Major advances in our understanding of the functional heterogeneity of enteric neurons are driven by the application of newly developed, innovative methods. In contrast to this progress, both animal and human enteric neurons are usually divided into only two morphological subpopulations, “Dogiel type II” neurons (with several long processes) and “Dogiel type I” neurons (with several short processes). This implies no more than the distinction of intrinsic primary afferent from all other enteric neurons. The well-known chemical and functional diversity of enteric neurons is not reflected by this restrictive dichotomy of morphological data. Recent structural investigations of human enteric neurons were performed by different groups which mainly used two methodical approaches, namely detecting the architecture of their processes and target-specific tracing of their axonal courses. Both methods were combined with multiple immunohistochemistry in order to decipher neurochemical codes. This review integrates these morphological and immunohistological data and presents a classification of human enteric neurons which we believe is not yet complete but provides an essential foundation for the further development of human gastrointestinal neuropathology.

Autonomic neuromuscular junctions

This review traces the history of the discovery and subsequent understanding of smooth muscle cells and their motor innervation. Smooth muscle tissue is made up of thousands of very small, individual, electrically connected, muscle cells. Each axon that enters a smooth muscle tissue branches extensively to form a terminal arbour that comes close to hundreds of smooth muscle cells. The branches of the terminal arbour are varicose, and each varicosity, of which there can be thousands, contains numerous transmitter storage vesicles. However, the probability of an individual varicosity releasing transmitter onto the adjacent muscle cells when an action potential passes is low. Many axons influence each muscle cell, some because they release transmitter close to the cell, and some because the events that they cause in other cells are electrically coupled to the cell under investigation. In tissues where this has been assessed, 20 or more axons can influence a single smooth muscle cell. We present a model of the innervation and influence of neurons on smooth muscle.

scRNA-Seq Reveals New Enteric Nervous System Roles for GDNF, NRTN, and TBX3

BACKGROUND AND AIMS

Bowel function requires coordinated activity of diverse enteric neuron subtypes. Our aim was to define gene expression in these neuron subtypes to facilitate development of novel therapeutic approaches to treat devastating enteric neuropathies, and to learn more about enteric nervous system function.

METHODS

To identify subtype-specific genes, we performed single-nucleus RNA-seq on adult mouse and human colon myenteric plexus, and single-cell RNA-seq on E17.5 mouse ENS cells from whole bowel. We used immunohistochemistry, select mutant mice, and calcium imaging to validate and extend results.

RESULTS

RNA-seq on 635 adult mouse colon myenteric neurons and 707 E17.5 neurons from whole bowel defined seven adult neuron subtypes, eight E17.5 neuron subtypes and hundreds of differentially expressed genes. Manually dissected human colon myenteric plexus yielded RNA-seq data from 48 neurons, 3798 glia, 5568 smooth muscle, 377 interstitial cells of Cajal, and 2153 macrophages. Immunohistochemistry demonstrated differential expression for BNC2, PBX3, SATB1, RBFOX1, TBX2, and TBX3 in enteric neuron subtypes. Conditional Tbx3 loss reduced NOS1-expressing myenteric neurons. Differential Gfra1 and Gfra2 expression coupled with calcium imaging revealed that GDNF and neurturin acutely and differentially regulate activity of ∼50% of myenteric neurons with distinct effects on smooth muscle contractions.

CONCLUSION

Single cell analyses defined genes differentially expressed in myenteric neuron subtypes and new roles for TBX3, GDNF and NRTN. These data facilitate molecular diagnostic studies and novel therapeutics for bowel motility disorders.

Advances in colonic motor complexes in mice

The primary functions of the gastrointestinal (GI) tract are to absorb nutrients, water, and electrolytes that are essential for life. This is accompanied by the capability of the GI tract to mix ingested content to maximize absorption and effectively excrete waste material. There have been major advances in understanding intrinsic neural mechanisms involved in GI motility. This review highlights major advances over the past few decades in our understanding of colonic motor complexes (CMCs), the major intrinsic neural patterns that control GI motility. CMCs are generated by rhythmic coordinated firing of large populations of myenteric neurons. Initially, it was thought that serotonin release from the mucosa was required for CMC generation. However, careful experiments have now shown that neither the mucosa nor endogenous serotonin are required, although, evidence suggests enteroendocrine (EC) cells modulate CMCs. The frequency and extent of propagation of CMCs are highly dependent on mechanical stimuli (circumferential stretch). In summary, the isolated mouse colon emerges as a good model to investigate intrinsic mechanisms underlying colonic motility and provides an excellent preparation to explore potential therapeutic agents on colonic motility, in a highly controlled in vitro environment. In addition, during CMCs, the mouse colon facilitates investigations into the emergence of dynamic assemblies of extensive neural networks, applicable to the nervous system of different organisms.

Diversification of molecularly defined myenteric neuron classes revealed by single-cell RNA sequencing

Autonomous regulation of the intestine requires the combined activity of functionally distinct neurons of the enteric nervous system (ENS). However, the variety of enteric neuron types and how they emerge during development remain largely unknown. Here, we define a molecular taxonomy of twelve enteric neuron classes within the myenteric plexus of the mouse small intestine using single cell RNA-sequencing. We present cell-cell communication features, histochemical markers for motor, sensory, and interneurons together with transgenic tools for class-specific targeting. Transcriptome analysis of embryonic ENS uncovers a novel principle of neuronal diversification, where two neuron classes arise through a binary neurogenic branching, and all other identities emerge through subsequent post-mitotic differentiation. We identify generic and class-specific transcriptional regulators and functionally connect Pbx3 to a post-mitotic fate transition. Our results offer a conceptual and molecular resource for dissecting ENS circuits, and predicting key regulators for directed differentiation of distinct enteric neuron classes.

A Novel Mode of Sympathetic Reflex Activation Mediated by the Enteric Nervous System

Enteric viscerofugal neurons provide a pathway by which the enteric nervous system (ENS), otherwise confined to the gut wall, can activate sympathetic neurons in prevertebral ganglia. Firing transmitted through these pathways is currently considered fundamentally mechanosensory. The mouse colon generates a cyclical pattern of neurogenic contractile activity, called the colonic motor complex (CMC). Motor complexes involve a highly coordinated firing pattern in myenteric neurons with a frequency of ∼2 Hz. However, it remains unknown how viscerofugal neurons are activated and communicate with the sympathetic nervous system during this naturally-occurring motor pattern. Here, viscerofugal neurons were recorded extracellularly from rectal nerve trunks in isolated tube and flat-sheet preparations of mouse colon held at fixed circumferential length. In freshly dissected preparations, motor complexes were associated with bursts of viscerofugal firing at 2 Hz that aligned with 2-Hz smooth muscle voltage oscillations. This behavior persisted during muscle paralysis with nicardipine. Identical recordings were made after a 4- to 5-d organotypic culture during which extrinsic nerves degenerated, confirming that recordings were from viscerofugal neurons. Single unit analysis revealed the burst firing pattern emerging from assemblies of viscerofugal neurons differed from individual neurons, which typically made partial contributions, highlighting the importance and extent of ENS-mediated synchronization. Finally, sympathetic neuron firing was recorded from the central nerve trunks emerging from the inferior mesenteric ganglion. Increased sympathetic neuron firing accompanied all motor complexes with a 2-Hz burst pattern similar to viscerofugal neurons. These data provide evidence for a novel mechanism of sympathetic reflex activation derived from synchronized firing output generated by the ENS.

Antibiotic exposure postweaning disrupts the neurochemistry and function of enteric neurons mediating colonic motor activity

The period during and immediately after weaning is an important developmental window when marked shifts in gut microbiota can regulate the maturation of the enteric nervous system (ENS). Because microbiota-derived signals that modulate ENS development are poorly understood, we examined the physiological impact of the broad spectrum of antibiotic, vancomycin-administered postweaning on colonic motility, neurochemistry of enteric neurons, and neuronal excitability. The functional impact of vancomycin on enteric neurons was investigated by Ca²⁺ imaging in Wnt1-Cre;R26R-GCaMP3 reporter mice to characterize alterations in the submucosal and the myenteric plexus, which contains the neuronal circuitry controlling gut motility. 16S rDNA sequencing of fecal specimens after oral vancomycin demonstrated significant deviations in microbiota abundance, diversity, and community composition. Vancomycin significantly increased the relative family rank abundance of Akkermansiaceae, Lactobacillaceae, and Enterobacteriaceae at the expense of Lachnospiraceae and Bacteroidaceae. In sharp contrast to neonatal vancomycin exposure, microbiota compositional shifts in weaned animals were associated with slower colonic migrating motor complexes (CMMCs) without mucosal serotonin biosynthesis being altered. The slowing of CMMCs is linked to disruptions in the neurochemistry of the underlying enteric circuitry. This included significant reductions in cholinergic and calbindin+ myenteric neurons, neuronal nitric oxide synthase+ submucosal neurons, neurofilament M+ enteric neurons, and increased proportions of cholinergic submucosal neurons. The antibiotic treatment also increased transmission and responsiveness in myenteric and submucosal neurons that may enhance inhibitory motor pathways, leading to slower CMMCs. Differential vancomycin responses during neonatal and weaning periods in mice highlight the developmental-specific impact of antibiotics on colonic enteric circuitry and motility.

Intrinsic sensory neurons provide direct input to motor neurons and interneurons in mouse distal colon via varicose baskets

Connections from intrinsic primary afferent neurons (IPANs), to ascending motor and interneurons have been described in guinea pig colon. These mono- and polysynaptic circuits may underlie polarized motor reflexes evoked by local gut stimulation. There is a need to translate findings in guinea pig to mouse, a species increasingly used in enteric neuroscience. Here, mouse distal colon was immunolabeled for CGRP, a marker of putative IPANs. This revealed a combination of large, intensely immunofluorescent axons in myenteric plexus and circular muscle, and thinner varicose axons with less immunofluorescence. The latter formed dense, basket-like varicosity clusters (CGRP+ baskets) that enveloped myenteric nerve cell bodies. Immunolabeling after 4-5 days in organ culture caused loss of large CGRP+ axons, but not varicose CGRP+ fibers and CGRP+ baskets. Baskets were characterized further by triple labeling with CGRP, nitric oxide synthase (NOS) and calretinin (CALR) antibodies. Approximately half (48%) of nerve cell bodies inside CGRP+ baskets lacked both NOS and CALR, while two overlapping populations containing NOS and/or CALR comprised the remainder. Quantitative analysis revealed CGRP+ varicosities were most abundant in baskets, followed by CALR+ varicosities, with a high degree of colocalization between the two markers. Few NOS+ varicosities occurred in baskets. Significantly higher proportions of CALR+ and CGRP+ varicosities colocalized in baskets than in circular muscle. In conclusion, CGRP+ baskets in mouse colon are formed by intrinsic enteric neurons with a neurochemical profile consistent with IPANs and have direct connections to both excitatory and inhibitory neurons.

2-Aminoethyldiphenyl Borinate: A Multitarget Compound with Potential as a Drug Precursor

BACKGROUND

Boron is considered a trace element that induces various effects in systems of the human body. However, each boron-containing compound exerts different effects.

OBJECTIVE

To review the effects of 2-Aminoethyldiphenyl borinate (2-APB), an organoboron compound, on the human body, but also, its effects in animal models of human disease.

METHODS

In this review, the information to showcase the expansion of these reported effects through interactions with several ion channels and other receptors has been reported. These effects are relevant in the biomedical and chemical fields due to the application of the reported data in developing therapeutic tools to modulate the functions of the immune, cardiovascular, gastrointestinal and nervous systems.

RESULTS

Accordingly, 2-APB acts as a modulator of adaptive and innate immunity, including the production of cytokines and the migration of leukocytes. Additionally, reports show that 2-APB exerts effects on neurons, smooth muscle cells and cardiomyocytes, and it provides a cytoprotective effect by the modulation and attenuation of reactive oxygen species.

CONCLUSION

The molecular pharmacology of 2-APB supports both its potential to act as a drug and the desirable inclusion of its moieties in new drug development. Research evaluating its efficacy in treating pain and specific maladies, such as immune, cardiovascular, gastrointestinal and neurodegenerative disorders, is scarce but interesting.

Endogenous Glutamate Excites Myenteric Calbindin Neurons by Activating Group I Metabotropic Glutamate Receptors in the Mouse Colon

Glutamate is a classic excitatory neurotransmitter in the central nervous system (CNS), but despite several studies reporting the expression of glutamate together with its various receptors and transporters within the enteric nervous system (ENS), its role in the gut remains elusive. In this study, we characterized the expression of the vesicular glutamate transporter, vGluT2, and examined the function of glutamate in the myenteric plexus of the distal colon by employing calcium (Ca²⁺)-imaging on Wnt1-Cre; R26R-GCaMP3 mice which express a genetically encoded fluorescent Ca²⁺ indicator in all enteric neurons and glia. Most vGluT2 labeled varicosities contained the synaptic vesicle release protein, synaptophysin, but not vesicular acetylcholine transporter, vAChT, which labels vesicles containing acetylcholine, the primary excitatory neurotransmitter in the ENS. The somata of all calbindin (calb) immunoreactive neurons examined received close contacts from vGluT2 varicosities, which were more numerous than those contacting nitrergic neurons. Exogenous application of L-glutamic acid (L-Glu) and N-methyl-D-aspartate (NMDA) transiently increased the intracellular Ca²⁺ concentration [Ca²⁺]_i in about 25% of myenteric neurons. Most L-Glu responsive neurons were calb immunoreactive. Blockade of NMDA receptors with APV significantly reduced the number of neurons responsive to L-Glu and NMDA, thus showing functional expression of NMDA receptors on enteric neurons. However, APV resistant responses to L-Glu and NMDA suggest that other glutamate receptors were present. APV did not affect [Ca²⁺]_i transients evoked by electrical stimulation of interganglionic nerve fiber tracts, which suggests that NMDA receptors are not involved in synaptic transmission. The group I metabotropic glutamate receptor (mGluR) antagonist, PHCCC, significantly reduced the amplitude of [Ca²⁺]_i transients evoked by a 20 pulse (20 Hz) train of electrical stimuli in L-Glu responsive neurons. This stimulus is known to induce slow synaptic depolarizations. Further, some neurons that had PHCCC sensitive [Ca²⁺]_i transients were calb immunoreactive and received vGluT2 varicosities. Overall, we conclude that electrically evoked release of endogenous glutamate mediates slow synaptic transmission via activation of group I mGluRs expressed by myenteric neurons, particularly those immunoreactive for calb.

Optogenetic control of the enteric nervous system and gastrointestinal transit

There are limited effective therapies available for improving gastrointestinal (GI) transit in mammals with intractable or chronic constipation. Current therapeutics to improve GI-transit usually require oral ingestion of therapeutic drugs, such as the serotonin receptor agonist prucalopride. However, most receptors are distributed all over the body and unsurprisingly drugs like prucalopride stimulate multiple organs, often leading to unwanted side effects. There is a desperate need in the community to improve GI-transit selectively without effects on other organs. Areas covered: We performed a systematic review of the literature on Pubmed and report significant technical advances in optogenetic control of the GI-tract. We discuss recent demonstrations that optogenetics can be used to potently control the activity of subsets of enteric neurons. Special focus is made of the first recent demonstration that wireless optogenetics can be used to stimulate the colon in conscious, freely-moving, untethered mice causing a significant increase in fecal pellet output. This is a significant technical breakthrough with a major therapeutic potential application to improve GI-transit. Expert opinion: The ability to selectively stimulate the ENS to modulate GI-transit in live mammals using light, avoids the need for oral consumption of any drugs and side effects; by stimulating only the GI-tract.

Regional complexity in enteric neuron wiring reflects diversity of motility patterns in the mouse large intestine

The enteric nervous system controls a variety of gastrointestinal functions including intestinal motility. The minimal neuronal circuit necessary to direct peristalsis is well-characterized but several intestinal regions display also other motility patterns for which the underlying circuits and connectivity schemes that coordinate the transition between those patterns are poorly understood. We investigated whether in regions with a richer palette of motility patterns, the underlying nerve circuits reflect this complexity. Using Ca²⁺ imaging, we determined the location and response fingerprint of large populations of enteric neurons upon focal network stimulation. Complemented by neuronal tracing and volumetric reconstructions of synaptic contacts, this shows that the multifunctional proximal colon requires specific additional circuit components as compared to the distal colon, where peristalsis is the predominant motility pattern. Our study reveals that motility control is hard-wired in the enteric neural networks and that circuit complexity matches the motor pattern portfolio of specific intestinal regions.

Netrin-1-like-immunoreactivity Coexpresses With DCC and Has a Differential Level in the Myenteric Cholinergic and Nitrergic Neurons of the Adult Mouse Colon

Netrin-1 is a potent axonal and neuronal guidance cue in the developing nervous system. Netrin-1 functions are mediated by its receptors, such as deleted in colorectal cancer (DCC) present on axons and neurons. Localization of DCC and Netrin-1 on various types of enteric neurons and their role in the mature enteric nervous system is unknown. The results of our study revealed that almost all enteric neurons and processes express DCC and Netrin-1 in the adult mice. Netrin-1-like-immunoreactivity (IR) was detected in the cytoplasm of neurons with some showing strong or weak staining. The majority of Netrin-1-like-immunoreactive enteric neurons were choline acetyltransferase (ChAT)-positive. However, ~19% of neurons were strongly Netrin-1-like-positive but ChAT-negative while ~8% of neurons were Netrin-1-like-negative but strongly ChAT-positive. In contrast, almost all nitric oxide synthase (nNOS)-positive enteric neurons displayed strong Netrin-1-like-IR. This differential intensity of Netrin-1 expression in the myenteric neurons might determine major neuronal subtypes regulating intestinal motility, ChAT-IR excitatory, and nNOS-IR inhibitory muscle motor and interneurons. This is the first study demonstrating the localization of DCC and Netrin-1 in the colonic myenteric plexus of the adult mice and their expression level determining two major neuronal subtypes regulating intestinal motility.

Identification of a Rhythmic Firing Pattern in the Enteric Nervous System That Generates Rhythmic Electrical Activity in Smooth Muscle

The enteric nervous system (ENS) contains millions of neurons essential for organization of motor behavior of the intestine. It is well established that the large intestine requires ENS activity to drive propulsive motor behaviors. However, the firing pattern of the ENS underlying propagating neurogenic contractions of the large intestine remains unknown. To identify this, we used high-resolution neuronal imaging with electrophysiology from neighboring smooth muscle. Myoelectric activity underlying propagating neurogenic contractions along murine large intestine [also referred to as colonic migrating motor complexes, (CMMCs)] consisted of prolonged bursts of rhythmic depolarizations at a frequency of ∼2 Hz. Temporal coordination of this activity in the smooth muscle over large spatial fields (∼7 mm, longitudinally) was dependent on the ENS. During quiescent periods between neurogenic contractions, recordings from large populations of enteric neurons, in mice of either sex, revealed ongoing activity. The onset of neurogenic contractions was characterized by the emergence of temporally synchronized activity across large populations of excitatory and inhibitory neurons. This neuronal firing pattern was rhythmic and temporally synchronized across large numbers of ganglia at ∼2 Hz. ENS activation preceded smooth muscle depolarization, indicating rhythmic depolarizations in smooth muscle were controlled by firing of enteric neurons. The cyclical emergence of temporally coordinated firing of large populations of enteric neurons represents a unique neural motor pattern outside the CNS. This is the first direct observation of rhythmic firing in the ENS underlying rhythmic electrical depolarizations in smooth muscle. The pattern of neuronal activity we identified underlies the generation of CMMCs.SIGNIFICANCE STATEMENT How the enteric nervous system (ENS) generates neurogenic contractions of smooth muscle in the gastrointestinal (GI) tract has been a long-standing mystery in vertebrates. It is well known that myogenic pacemaker cells exist in the GI tract [called interstitial cells of Cajal (ICCs)] that generate rhythmic myogenic contractions. However, the mechanisms underlying the generation of rhythmic neurogenic contractions of smooth muscle in the GI tract remains unknown. We developed a high-resolution neuronal imaging method with electrophysiology to address this issue. This technique revealed a novel pattern of rhythmic coordinated neuronal firing in the ENS that has never been identified. Rhythmic neuronal firing in the ENS was found to generate rhythmic neurogenic depolarizations in smooth muscle that underlie contraction of the GI tract.

Neurally Released GABA Acts via GABAC Receptors to Modulate Ca2+ Transients Evoked by Trains of Synaptic Inputs, but Not Responses Evoked by Single Stimuli, in Myenteric Neurons of Mouse Ileum

γ-Aminobutyric Acid (GABA) and its receptors, GABAA,B,C, are expressed in several locations along the gastrointestinal tract. Nevertheless, a role for GABA in enteric synaptic transmission remains elusive. In this study, we characterized the expression and function of GABA in the myenteric plexus of the mouse ileum. About 8% of all myenteric neurons were found to be GABA-immunoreactive (GABA+) including some Calretinin+ and some neuronal nitric oxide synthase (nNOS+) neurons. We used Wnt1-Cre;R26R-GCaMP3 mice, which express a genetically encoded fluorescent calcium indicator in all enteric neurons and glia. Exogenous GABA increased the intracellular calcium concentration, [Ca2+]i of some myenteric neurons including many that did not express GABA or nNOS (the majority), some GABA+, Calretinin+ or Neurofilament-M (NFM)+ but rarely nNOS+ neurons. GABA+ terminals contacted a significantly larger proportion of the cell body surface area of Calretinin+ neurons than of nNOS+ neurons. Numbers of neurons with GABA-induced [Ca2+]i transients were reduced by GABAA,B,C and nicotinic receptor blockade. Electrical stimulation of interganglionic fiber tracts was used to examine possible effects of endogenous GABA release. [Ca2+]i transients evoked by single pulses were unaffected by specific antagonists for each of the 3 GABA receptor subtypes. [Ca2+]i transients evoked by 20 pulse trains were significantly amplified by GABAC receptor blockade. These data suggest that GABAA and GABAB receptors are not involved in synaptic transmission, but suggest a novel role for GABAC receptors in modulating slow synaptic transmission, as indicated by changes in [Ca2+]i transients, within the ENS.

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.

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.

The Diverse Metabolic Roles of Peripheral Serotonin

Serotonin (5-hydroxytryptamine or 5-HT) is a multifunctional bioamine with important signaling roles in a range of physiological pathways. Almost all of the 5-HT in our bodies is synthesized in specialized enteroendocrine cells within the gastrointestinal (GI) mucosa called enterochromaffin (EC) cells. These cells provide all of our circulating 5-HT. We have long appreciated the important contributions of 5-HT within the gut, including its role in modulating GI motility. However, evidence of the physiological and clinical significance of gut-derived 5-HT outside of the gut has recently emerged, implicating 5-HT in regulation of glucose homeostasis, lipid metabolism, bone density, and diseases associated with metabolic syndrome, such as obesity and type 2 diabetes. Although a new picture has developed in the last decade regarding the various metabolic roles of peripheral serotonin, so too has our understanding of the physiology of EC cells. Given that they are scattered throughout the lining of the GI tract within the epithelial cell layer, these cells are typically difficult to study. Advances in isolation procedures now allow the study of pure EC-cell cultures and single cells, enabling studies of EC-cell physiology to occur. EC cells are sensory cells that are capable of integrating cues from ingested nutrients, the enteric nervous system, and the gut microbiome. Thus, levels of peripheral 5-HT can be modulated by a multitude of factors, resulting in both local and systemic effects for the regulation of a raft of physiological pathways related to metabolism and obesity.

Optogenetic Demonstration of Functional Innervation of Mouse Colon by Neurons Derived From Transplanted Neural Cells

BACKGROUND & AIMS

Cell therapy offers the potential to treat gastrointestinal motility disorders caused by diseased or absent enteric neurons. We examined whether neurons generated from transplanted enteric neural cells provide a functional innervation of bowel smooth muscle in mice.

METHODS

Enteric neural cells expressing the light-sensitive ion channel, channelrhodopsin, were isolated from the fetal or postnatal mouse bowel and transplanted into the distal colon of 3- to 4-week-old wild-type recipient mice. Intracellular electrophysiological recordings of responses to light stimulation of the transplanted cells were made from colonic smooth muscle cells in recipient mice. Electrical stimulation of endogenous enteric neurons was used as a control.

RESULTS

The axons of graft-derived neurons formed a plexus in the circular muscle layer. Selective stimulation of graft-derived cells by light resulted in excitatory and inhibitory junction potentials, the electrical events underlying contraction and relaxation, respectively, in colonic muscle cells. Graft-derived excitatory and inhibitory motor neurons released the same neurotransmitters as endogenous motor neurons-acetylcholine and a combination of adenosine triphosphate and nitric oxide, respectively. Graft-derived neurons also included interneurons that provided synaptic inputs to motor neurons, but the pharmacologic properties of interneurons varied with the age of the donors from which enteric neural cells were obtained.

CONCLUSIONS

Enteric neural cells transplanted into the bowel give rise to multiple functional types of neurons that integrate and provide a functional innervation of the smooth muscle of the bowel wall. Circuits composed of both motor neurons and interneurons were established, but the age at which cells are isolated influences the neurotransmitter phenotype of interneurons that are generated.

Motility patterns in mouse colon: gastrointestinal dysfunction induced by anticancer chemotherapy

Colon cancer is a leading cause of cancer-related death in humans. 5-Fluorouracil (5-FU), a major chemotherapy treatment, has been used for decades to fight numerous types of cancers, including breast, colon, and head and neck carcinomas. Unfortunately, a large proportion of patients treated with 5-FU develop toxicities that include diarrhea, mucositis, neutropenia, and vomiting. While the side effects of 5-FU are well known, the mechanisms underlying the induction of these unpleasant symptoms are poorly understood. The study by McQuade et al. in this issue of Neurogastroenterology & Motility provides important new potential explanations for the gastrointestinal (GI) dysfunction induced by 5-FU. These researchers carefully investigated an overlooked research area in which the symptoms of GI-motility dysfunction maybe due to an effect on the enteric nervous system. McQuade et al. delivered 5-FU treatment to mice and discovered an initial increase in GI transit (associated with acute intestinal inflammation), followed by a slowing in transit. Major differences were noted in characteristics of colonic migrating motor complexes. These effects maybe causally related to deficits in enteric ganglia or neurotransmission. Their study identified specific neurochemical classes of neurons in the myenteric plexus most affected by 5-FU. This is the first study to provide evidence that the functional intrinsic neural pathways within the enteric nervous system are likely impaired by 5-FU, leading to colonic dysmotility. This review will describe major patterns of motor activity in isolated whole mouse colon and how these patterns are modified by anticancer chemotherapy.

Imaging activation of peptidergic spinal afferent varicosities within visceral organs using novel CGRPα-mCherry reporter mice

In vertebrates, visceral pain from internal organs is detected by spinal afferents, whose cell bodies lie in dorsal root ganglia (DRG). Until now, all recordings from spinal afferents have been restricted to recording transmission of action potentials along axons, or from cell bodies lying outside their target organ, which is not where sensory transduction occurs. Our aim was to record directly from a major class of spinal afferent within visceral organs, where transduction of sensory stimuli into action potentials occurs. Using novel calcitonin gene-related peptide (CGRP)α reporter mice, DRG neurons expressed mCherry, including nerve axons within viscera. In colon, a minority of total CGRP immunoreactivity was attributed CGRPα. In isolated unstretched colon, calcium imaging from CGRPα-expressing varicose axons did not detect resolvable calcium transients. However, noxious levels of maintained circumferential stretch to the colon induced repetitive calcium transients simultaneously in multiple neighboring varicosities along single mCherry-expressing axons. Discrete varicosities could generate unitary calcium transients independently of neighboring varicosities. However, axons expressing mCherry only generated coordinated calcium transients when accompanied by simultaneous activation of multiple varicosities along that axon. Simultaneous imaging from different classes of myenteric neurons at the same time as mCherry-expressing axons revealed coordinated calcium transients in multiple myenteric neurons, independent of activity in mCherry-expressing axons. CGRPα-expressing axon terminals preferentially responded to heat, capsaicin, and low pH. We show that direct recordings can be made from the major class of peptidergic spinal afferent that contributes to visceral nociception. This approach can provide powerful insights into transduction of stimuli in viscera.

The enteric nervous system in patients with calculous and acalculous gallbladder

BACKGROUND

It is generally thought that gallbladder motility plays a more or less important role in the pathogenesis of gallstones. Some studies have shown that some abnormalities of its intrinsic innervations, but these studies were usually limited to one cell component.

AIMS

We investigated the main cell components of gallbladder intrinsic innervation in patients with and without gallstones.

METHODS

Archival gallbladder specimens from 39 patients, 27 with gallstones (age range 45-69 yrs) and 12 patients without gallstones (age range 39-71 yrs) were obtained. Full thickness sections were obtained from the gallbladder neck and immunohistochemistry was carried out for enteric neurons (neuron-specific enolase and calretinin), enteric glia (S100) and interstitial cells of Cajal (CD117 and CD34); tryptase staining was also done to distinguish the latter from mast cells.

RESULTS

Apart from calretinin-positive neurons, patients with gallstones featured a significant decrease of neurons, enteric glial cells (EGC) and mast cells compared to those without gallstones; interstitial cells of Cajal were extremely few and only found in two patients, one for each group.

CONCLUSIONS

The intrinsic innervations of the gallbladder is abnormal in gallstone patients, and this may contribute to gallstone formation in these subjects.