The morphology and projections of retrogradely labeled myenteric neurons in the human intestine

Over 30 Years of DiI Use for Human Neuroanatomical Tract Tracing: A Scoping Review

In the present study, we conducted a scoping review to provide an overview of the existing literature on the carbocyanine dye DiI, in human neuroanatomical tract tracing. The PubMed, Scopus, and Web of Science databases were systematically searched. We identified 61 studies published during the last three decades. While studies incorporated specimens across human life from the embryonic stage onwards, the majority of studies focused on adult human tissue. Studies that utilized peripheral nervous system (PNS) tissue were a minority, with the majority of studies focusing on the central nervous system (CNS). The most common topic of interest in previous tract tracing investigations was the connectivity of the visual pathway. DiI crystals were more commonly applied. Nevertheless, several studies utilized DiI in a paste or dissolved form. The maximum tracing distance and tracing speed achieved was, respectively, 70 mm and 1 mm/h. We identified studies that focused on optimizing tracing efficacy by varying parameters such as fixation, incubation temperature, dye re-application, or the application of electric fields. Additional studies aimed at broadening the scope of DiI use by assessing the utility of archival tissue and compatibility of tissue clearing in DiI applications. A combination of DiI tracing and immunohistochemistry in double-labeling studies have been shown to provide the means for assessing connectivity of phenotypically defined human CNS and PNS neuronal populations.

Overview of the Enteric Nervous System

Propulsion of contents in the gastrointestinal tract requires coordinated functions of the extrinsic nerves to the gut from the brain and spinal cord, as well as the neuromuscular apparatus within the gut. The latter includes excitatory and inhibitory neurons, pacemaker cells such as the interstitial cells of Cajal and fibroblast-like cells, and smooth muscle cells. Coordination between these extrinsic and enteric neurons results in propulsive functions which include peristaltic reflexes, migrating motor complexes in the small intestine which serve as the housekeeper propelling to the colon the residual content after digestion, and mass movements in the colon which lead to defecation.

Types of Neurons in the Human Colonic Myenteric Plexus Identified by Multilayer Immunohistochemical Coding

BACKGROUND AND AIMS

Gut functions including motility, secretion, and blood flow are largely controlled by the enteric nervous system. Characterizing the different classes of enteric neurons in the human gut is an important step to understand how its circuitry is organized and how it is affected by disease.

METHODS

Using multiplexed immunohistochemistry, 12 discriminating antisera were applied to distinguish different classes of myenteric neurons in the human colon (2596 neurons, 12 patients) according to their chemical coding. All antisera were applied to every neuron, in multiple layers, separated by elutions.

RESULTS

A total of 164 combinations of immunohistochemical markers were present among the 2596 neurons, which could be divided into 20 classes, with statistical validation. Putative functions were ascribed for 4 classes of putative excitatory motor neurons (EMN1-4), 4 inhibitory motor neurons (IMN1-4), 3 ascending interneurons (AIN1-3), 6 descending interneurons (DIN1-6), 2 classes of multiaxonal sensory neurons (SN1-2), and a small, miscellaneous group (1.8% of total). Soma-dendritic morphology was analyzed, revealing 5 common shapes distributed differentially between the 20 classes. Distinctive baskets of axonal varicosities surrounded 45% of myenteric nerve cell bodies and were associated with close appositions, suggesting possible connectivity. Baskets of cholinergic terminals and several other types of baskets selectively targeted ascending interneurons and excitatory motor neurons but were significantly sparser around inhibitory motor neurons.

CONCLUSIONS

Using a simple immunohistochemical method, human myenteric neurons were shown to comprise multiple classes based on chemical coding and morphology and dense clusters of axonal varicosities were selectively associated with some classes.

How big is the little brain in the gut? Neuronal numbers in the enteric nervous system of mice, Guinea pig, and human

BACKGROUND

Despite numerous studies on the enteric nervous system (ENS), we lack fundamental knowledge on neuronal densities or total neuron numbers in different species. There are more anecdotal than actual figures on nerve counts.

METHODS

We used standardized preparation techniques and immunohistochemistry with validated panneuronal markers (human or mouse anti-HuD/C) to determine neuronal densities in specimen from the entire gastrointestinal tract of mice, guinea pig, and humans. In parallel, we measured the dimensions of the gastrointestinal regions in mouse and guinea pig. For humans, we had to rely on literature data.

KEY RESULTS

The average neuronal densities along the gastrointestinal tract were 35,011 ± 25,017 1/cm² for the myenteric and 16,685 ± 9098 1/cm² for the submucous plexus in mice, 24,315 ± 16,627 and 11,850 ± 6122 1/cm² for guinea pig myenteric and submucous plexus, respectively, and 21,698 ± 9492 and 16,367 ± 5655 1/cm² for human myenteric and submucous plexus, respectively. The total number of neurons in the ENS was 2.6 million for mice, 14.6 million for guinea pig, and 168 million for human.

CONCLUSIONS & INFERENCES

This study reports the first comprehensive nerve cell count in mice, guinea pig, and human ENS. Neuronal densities were comparable between the three species and the differences in the total numbers of enteric neurons are likely due to body size and intestinal length. The number of enteric neurons is comparable to the number of neurons in the spinal cord for all three species.

The human enteric nervous system. Historical and modern advances. Collaboration between science and surgery

BACKGROUND

There are considerable advantages and opportunities for surgeons and trainee surgeons in conducting a period of research allied with basic scientists. Such clinicians are well placed to define relevant clinical questions, provide human material (tissue, biopsy and blood) and translate the techniques derived in experimental animals to human subjects.

METHODS

This small review explores research conducted on the nervous system of the intestines, with an emphasis on the translation of findings from animal to human.

RESULTS

This work shows that new techniques of immunohistochemistry and retrograde tracing, developed in animal tissue, have greatly expanded our knowledge of the structure of the human enteric nervous system.

CONCLUSIONS

Such findings have sparked therapeutic trials for the treatment of gastrointestinal disorders in patients.

Identifying Types of Neurons in the Human Colonic Enteric Nervous System

Distinguishing and characterising the different classes of neurons that make up a neural circuit has been a long-term goal for many neuroscientists. The enteric nervous system is a large but moderately simple part of the nervous system. Enteric neurons in laboratory animals have been extensively characterised morphologically, electrophysiologically, by projections and immunohistochemically. However, studies of human enteric nervous system are less advanced despite the potential availability of tissue from elective surgery (with appropriate ethics permits). Recent studies using single cell sequencing have confirmed and extended the classification of enteric neurons in mice and human, but it is not clear whether an encompassing classification has been achieved. We present preliminary data on a means to distinguish classes of myenteric neurons in specimens of human colon combining immunohistochemical, morphological, projection and size data on single cells. A method to apply multiple layers of antisera to specimens was developed, allowing up to 12 markers to be characterised in individual neurons. Applied to multi-axonal Dogiel type II neurons, this approach demonstrated that they constitute fewer than 5% of myenteric neurons, are nearly all immunoreactive for choline acetyltransferase and tachykinins. Many express the calcium-binding proteins calbindin and calretinin and they are larger than average myenteric cells. This methodology provides a complementary approach to single-cell mRNA profiling to provide a comprehensive account of the types of myenteric neurons in the human colon.

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.

Characterization of putative interneurons in the myenteric plexus of human colon

BACKGROUND

The enteric nervous system contains multiple classes of neurons, distinguishable by morphology, immunohistochemical markers, and projections; however, specific combinations differ between species. Here, types of enteric neurons in human colon were characterized immunohistochemically, using retrograde tracing combined with multiple labeling immunohistochemistry, focussing on non-motor neurons.

METHODS

The fluorescent carbocyanine tracer, DiI, was applied to the myenteric plexus in ex vivo preparations, filling neurons projecting within the plexus. Limits of projection lengths of motor neurons were established, allowing them to be excluded from the analysis. Long ascending and descending interneurons were then distinguished by labeling for discriminating immunohistochemical markers: calbindin, calretinin, enkephalin, 5-hydroxytryptamine, nitric oxide synthase, and substance P. These results were combined with a previous published study in which nitric oxide synthase and choline acetyltransferase immunoreactivities were established.

KEY RESULTS

Long ascending neurons (with projections longer than 8 mm, which excludes more than 95% motor neurons) formed four types, in descending order of abundance, defined by immunoreactivity for: (a) ChAT+/ENK+, (b) ChAT+/ENK+/SP+, (c) ChAT+/Calb+, and (d) ChAT+/ENK+/Calb+. Long descending neurons, up to 70 mm long also formed at least four types, distinguished by immunoreactivity for (a) NOS + cells (without ChAT), (b) ChAT+/NOS+, (c) ChAT+/Calret+, and (d) ChAT+/5HT + cells (with or without NOS).

CONCLUSIONS AND INFERENCES

Long interneurons, which do not innervate muscularis externa, are likely to coordinate neural activity over distances of many centimeters along the colon. Characterizing their neurochemical coding provides a basis for understanding their roles, investigating their connectivity, and building a comprehensive account of human colonic enteric neurons.

Distinct patterns of myogenic motor activity identified in isolated human distal colon with high-resolution manometry

BACKGROUND

Colonic high-resolution manometry (HRM) has been used to reveal discrete, propagating colonic motor patterns. To help determine mechanisms underlying these patterns, we used HRM to record contractile activity in human distal colon ex vivo.

METHODS

Surgically excised segments of descending (n = 30) or sigmoid colon (n = 4) were immersed in oxygenated Krebs solution at 36°C (n = 34; 16 female; 67.6 ± 12.4 years; length: 24.7 ± 3.5 cm). Contractility was recorded by HRM catheters. After 30 minutes of baseline recording, 0.3 mM lidocaine and/or 1 mM hexamethonium were applied. Ascending neural pathways were activated by electrical field stimulation (EFS; 10 Hz, 0.5 ms, 50 V, 5-s duration) applied to the anal end before and after drug application.

RESULTS

Spontaneous propagating contractions were recorded in all specimens (0.1-1.5 cycles/minute). Most contractions occurred synchronously across all recording sites. In five specimens, rhythmic antegrade contractions propagated across the full length of the preparation. EFS evoked local contractions at the site of stimulation (latency: 5.5 ± 2.4 seconds) with greater amplitude than spontaneous contractions (EFS; 29.3 ± 26.9 vs 12.1 ± 14.8 mm Hg; P = .02). Synchronous or retrograde propagating motor patterns followed EFS; 71% spanned the entire preparation length. Hexamethonium and lidocaine modestly and only temporarily inhibited spontaneous contractions, whereas TTX increased the frequency of contractile activity while inhibiting EFS-evoked contractions.

CONCLUSIONS AND INFERENCES

Our study suggests that the propagated contractions recorded in the organ bath have a myogenic origin which can be regulated by neural input. Once activated at a local site, the contractions do not require the propulsion of fecal content to sustain long-distance propagation.

Characterization of projections of longitudinal muscle motor neurons in human colon

BACKGROUND

The enteric nervous system contains inhibitory and excitatory motor neurons which modulate smooth muscle contractility. Cell bodies of longitudinal muscle motor neurons have not been identified in human intestine.

METHODS

We used retrograde tracing ex vivo with DiI, with multiple labeling immunohistochemistry, to characterize motor neurons innervating tenial and inter-tenial longitudinal muscle of human colon.

KEY RESULTS

The most abundant immunohistochemical markers in the tertiary plexus were vesicular acetylcholine transporter, nitric oxide synthase (NOS), and vasoactive intestinal polypeptide (VIP). Of retrogradely traced motor neurons innervating inter-tenial longitudinal muscle, 95% were located within 6mm oral or anal to the DiI application site. Excitatory motor neuron cell bodies, immunoreactive for choline acetyltransferase (ChAT), were clustered aborally, whereas NOS-immunoreactive cell bodies were distributed either side of the DiI application site. Motor neurons had small cell bodies, averaging 438 + 18µm² in cross-sectional area, similar for ChAT- and NOS-immunoreactive subtypes. Motor neurons innervating the tenia had slightly longer axial projections, with 95% located within 9mm. ChAT-immunoreactive excitatory motor neurons to tenia were clustered aborally, whereas NOS-immunoreactive inhibitory motor neurons had both ascending and descending projections. VIP immunoreactivity was rarely present without NOS immunoreactivity in motor neurons.

CONCLUSIONS AND INFERENCES

Tenial and inter-tenial motor neurons innervating the longitudinal muscle have short projections. Inhibitory motor neurons have less polarized projections than cholinergic excitatory motor neurons. Longitudinal and circular muscle layers are innervated by distinct local populations of excitatory and inhibitory motor neurons. A population of human enteric neurons that contribute significantly to colonic motility has been characterized.

The Enteric Nervous System for Epithelial Researchers: Basic Anatomy, Techniques, and Interactions With the Epithelium

The intestinal epithelium does not function in isolation, but interacts with many components including the Enteric Nervous System (ENS). Understanding ENS and intestinal epithelium interactions requires multidisciplinary approaches to uncover cells involved, mechanisms used, and the ultimate influence on intestinal physiology. This review is intended to serve as a reference for epithelial biologists interested in studying these interactions. With this in mind, this review aims to summarize the basic anatomy of the epithelium and ENS, mechanisms by which they interact, and techniques used to study these interactions. We highlight in vitro, ex vivo and in vivo techniques. Additionally, ENS influence on epithelial proliferation and gene expression within stem and differentiated cells as well as gastrointestinal cancer are discussed.

Distribution, projections, and association with calbindin baskets of motor neurons, interneurons, and sensory neurons in guinea-pig distal colon

Normal gut function relies on the activity of the enteric nervous system (ENS) found within the wall of the gastrointestinal tract. The structural and functional organization of the ENS has been extensively studied in the guinea pig small intestine, but less is known about colonic circuitry. Given that there are significant differences between these regions in function, observed motor patterns and pathology, it would be valuable to have a better understanding of the colonic ENS. Furthermore, disorders of colonic motor function, such as irritable bowel syndrome, are much more common. We have recently reported specialized basket-like structures, immunoreactive for calbindin, that likely underlie synaptic inputs to specific types of calretinin-immunoreactive neurons in the guinea-pig colon. Based on detailed immunohistochemical analysis, we postulated the recipient neurons may be excitatory motor neurons and ascending interneurons. In the present study, we combined retrograde tracing and immunohistochemistry to examine the projections of circular muscle motor neurons, myenteric interneurons, and putative sensory neurons. We focused on neurons with immunoreactivity for calbindin, calretinin and nitric oxide synthase and their relationship with calbindin baskets. Retrograde tracing using indocarbocyanine dye (DiI) revealed that many of the nerve cell bodies surrounded by calbindin baskets belong to motor neurons and ascending interneurons. Unique functional classes of myenteric neurons were identified based on morphology, neuronal markers and polarity of projection. We provide evidence for three groups of ascending motor neurons based on immunoreactivity and association with calbindin baskets, a finding that may have significant functional implications.

Chronic constipation

Chronic constipation is a prevalent condition that severely impacts the quality of life of those affected. Several types of primary chronic constipation, which show substantial overlap, have been described, including normal-transit constipation, rectal evacuation disorders and slow-transit constipation. Diagnosis of primary chronic constipation involves a multistep process initiated by the exclusion of 'alarm' features (for example, unintentional weight loss or rectal bleeding) that might indicate organic diseases (such as polyps or tumours) and a therapeutic trial with first-line treatments such as dietary changes, lifestyle modifications and over-the-counter laxatives. If symptoms do not improve, investigations to diagnose rectal evacuation disorders and slow-transit constipation are performed, such as digital rectal examination, anorectal structure and function testing (including the balloon expulsion test, anorectal manometry or defecography) or colonic transit tests (such as the radiopaque marker test, wireless motility capsule test, scintigraphy or colonic manometry). The mainstays of treatment are diet and lifestyle interventions, pharmacological therapy and, rarely, surgery. This Primer provides an introduction to the epidemiology, pathophysiological mechanisms, diagnosis, management and quality of life associated with the commonly encountered clinical problem of chronic constipation in adults unrelated to opioid abuse.

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.

Ascending excitatory neural pathways modulate slow phasic myogenic contractions in the isolated human colon

BACKGROUND

In animal models, enteric reflex pathways have potent effects on motor activity; their roles have been much less extensively studied in human gut. The aim of this study was to determine if ascending excitatory interneuronal pathways can modulate spontaneous phasic contractions in isolated preparations of human colonic circular muscle.

METHODS

Human colonic preparations were cut into T shapes, with vertical bar of the 'T' pharmacologically isolated. Electrical stimulation and the nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), were applied to the isolated region and circular muscle contractile activity was measured from the cross-bar of the T, more than 10 mm orally from the region of stimulation.

KEY RESULTS

The predominant form of spontaneous muscle activity consisted of tetrodotoxin-resistant, large amplitude, slow phasic contractions (SPCs), occurring at average intervals of 124 ± 68 s. Addition of a high concentration of hexamethonium (1 mmol L(-1)) to the superfusing solution significantly increased the interval between SPCs to 278.1 ± 138.3 s (P < 0.005). Focal electrical stimulation more than 10 mm aboral to the muscle recording site advanced the onset of the next SPC, and this effect persisted in hexamethonium. However, the effect of electrical stimulation was blocked by tetrodotoxin (TTX, 1 μmol L(-1)). Application of the nicotinic agonist DMPP (1 mmol L(-1)) to the aboral chamber often stimulated a premature SPC (n = 4).

CONCLUSIONS & INFERENCES

The major form of spontaneous contractility in preparations of human colonic circular muscle is SPCs, which are myogenic in origin. Activation of ascending excitatory neural pathways, which involve nicotinic receptors, can modulate the timing of SPCs and thus influence human colonic motility.

Calretinin and somatostatin immunoreactivities label different human submucosal neuron populations

In human myenteric plexus, calretinin (CALR) and somatostatin (SOM) coexist in Dogiel Type II neurons, which were considered as intrinsic primary afferent neurons in the guinea pig. The aims of this study were to test if also human submucosal neurons costain immunohistochemically for CALR and SOM and whether these or other neurons display Type II morphology. Two sets of submucosal wholemounts of small and large intestine from 29 patients (median age 65 years) were triple stained for CALR, SOM, and human neuronal protein Hu C/D (HU, a pan-neuronal marker) as well as for CALR, SOM, and peripherin (PER), respectively. Only exceptionally, neurons coreactive for both CALR and SOM were found. The three major groups of neurons were CALR-/HU-coreactive (CALR-neurons), SOM-/HU-coreactive (SOM-neurons), and HU-alone-positive neurons. We observed significantly more CALR-neurons in the external submucosal plexus (ESP) of all regions and more SOM-neurons in the internal submucosal plexus (ISP), although with substantial interindividual variations. Comparisons of small vs. large intestine revealed more SOM-neurons (ESP: 29% vs. 4%, ISP: 40% vs. 13%) but fewer CALR-neurons (ESP: 37% vs. 77%, ISP: 21% vs. 67%) in small intestine. Morphologically, CALR-neurons had multiple processes; in some cases, we identified multidendritic/uniaxonal neurons. In contrast, SOM-neurons had mostly only one process. The functions of both populations as possible primary afferent neurons, interneurons, secretomotor neurons, or vasomotor neurons are discussed. Future morphochemical distinction of these groups may reveal different subgroups.

Partial, selective survival of nitrergic neurons in chagasic megacolon

One frequent chronic syndrome of Chagas’ disease is megacolon, an irreversible dilation of a colonic segment. Extensive enteric neuron loss in the affected segment is regarded as key factor for deficient motility. Here, we assessed the quantitative balance between cholinergic and nitrergic neurons representing the main limbs of excitatory and inhibitory colonic motor innervation, respectively. From surgically removed megacolonic segments of four patients, each three myenteric wholemounts (from non-dilated oral, megacolonic and non-dilated anal parts) was immunohistochemically triple-stained for choline acetyltransferase, neuronal nitric oxide synthase (NOS) and the panneuronal human neuronal protein Hu C/D. Degenerative changes were most pronounced in the megacolonic and anal regions, e.g. bulked, honeycomb-like ganglia with few neurons which were partly enlarged or atrophic or vacuolated. Neuron counts from each 15 ganglia of 12 megacolonic wholemounts were compared with those of 12 age- and region-matched controls. Extensive neuron loss, mainly in megacolonic and anal wholemounts, was obvious. In all three regions derived from megacolonic samples, the proportion of NOS-positive neurons (control: 55%) was significantly increased: in non-dilated oral parts to 61% (p = 0.003), in megacolonic regions to 72% (p < 0.001) and in non-dilated anal regions to 78% (p < 0.001). We suggest the chronic dilation of megacolonic specimens to be due to the preponderance of the nitrergic, inhibitory input to the intestinal muscle. However, the observed neuronal imbalance was not restricted to the dilated regions: the non-dilated anal parts may be innervated by ascending, cholinergic axons emerging from less affected, more anally located regions.

Neuroanatomy and physiology of colorectal function and defaecation: from basic science to human clinical studies

Colorectal physiology is complex and involves programmed, coordinated interaction between muscular and neuronal elements. Whilst a detailed understanding remains elusive, novel information has emerged from recent basic science and human clinical studies concerning normal sensorimotor mechanisms and the organization and function of the key elements involved in the control of motility. This chapter summarizes these observations to provide a contemporary review of the neuroanatomy and physiology of colorectal function and defaecation.

Role of nitric oxide produced by constitutive and inducible nitric oxide synthases in the mouse gastric fundus

In the present study, the role of nitric oxide (NO) produced by constitutive and inducible nitric oxide synthases (cNOS and iNOS, resepctively) on the contraction and relaxation of fundus in normal and lipopolysaccharide (LPS)-treated mice was examined. A whole fundic ring isolated from mice pretreated with reserpine was mounted in an organ bath containing Krebs' solution with 0.001 mmol/L atropine. Rings were contracted initially by 5-hydroxytryptamine (5-HT; 0.03 mmol/L) before relaxation was induced using ATP (0.03 mmol/L), ADP (0.03 mmol/L), pentoxifylline (0.002 mmol/L), electrical field stimulation (EFS; 50 V, 1 msec, 50 Hz, 3 min) and L-arginine (0.05 mmol/L). All drugs and EFS induced significant relaxation of isolated rings. The relaxations induced were significantly inhibited by N(G)-nitro-L-arginine methyl ester (L-NAME; 1.0 mmol/L). However, the iNOS inhibitors L-N(6)-(1-iminoethyl) lysine hydrochloride (L-NIL; 1.0 mmol/L) and amino guanidine (AMG; 1.0 mmol/L) had no significant effect on tissue relaxation. Then, the relaxant effects of 0.03 mmol/L ATP were tested on precontracted isolated fundic rings taken from 10 mg/kg LPS-treated animals. The non-selective NOS inhibitor L-NAME (10 mg/kg), the iNOS inhibitors L-NIL (3 mg/kg) and AMG (20 mg/kg) and betamethasone (0.1 mg/kg) were used to examine the role of NO produced by iNOS in the relaxation responses. It was found that the level of contraction induced by 0.03 mmol/L 5-HT in rings isolated from LPS-treated animals was significantly (P < 0.5) less than that in rings from untreated mice. However, precontracted tissues from LPS-treated mice were significantly relaxed by ATP and the relaxation response to ATP was significantly inhibited by L-NIL, ANG and betamethasone, but not by L-NAME. We suggest that, in LPS-treated mice, the production of NO from iNOS produces a reduction in the contractile response, as well as a decrease in NO formation by cNOS, resulting in changes to smooth muscle cell function.

Quantification of subclasses of human colonic myenteric neurons by immunoreactivity to Hu, choline acetyltransferase and nitric oxide synthase

An accurate method to count human enteric neurons is essential to develop a comprehensive account of the classes of nerve cells responsible for gut function and dysfunction. The majority of cells in the enteric nervous system utilize acetyl choline, or nitric oxide, or a combination of these, as neurotransmitters. Antisera raised against the RNA-binding protein Hu, were used to identify nerve cell bodies in whole mounts of the myenteric plexus of human colon, and then were utilized to analyse cells immunoreactive for combinations of choline acetyltransferase and nitric oxide synthase. Antisera to Hu provided a reliable means to count apparently all enteric nerve cell bodies, revealing 10% more cell bodies than labelling with neuron specific enolase, and no labelling of glial cells as revealed by S100. ChAT+/NOS- neurons accounted for 48% (+/-3%) of myenteric neurons and ChAT-/NOS+ neurons accounted for 43% (+/-2.5%). ChAT+/NOS+ neurons comprised 4% (+/-0.5) of the total number of neurons, and a novel class of small ChAT-/NOS- neurons, making up 5% (+/-0.9%) of all cells, was described for the first time.

Different responsiveness of excitatory and inhibitory enteric motor neurons in the human esophagus to electrical field stimulation and to nicotine

To compare electrical field stimulation (EFS) with nicotine in the stimulation of excitatory and inhibitory enteric motoneurons (EMN) in the human esophagus, circular lower esophageal sphincter (LES), and circular and longitudinal esophageal body (EB) strips from 20 humans were studied in organ baths. Responses to EFS or nicotine (100 microM) were compared in basal conditions, after N(G)-nitro-l-arginine (l-NNA; 100 microM), and after l-NNA and apamin (1 microM). LES strips developed myogenic tone enhanced by TTX (5 microM) or l-NNA. EFS-LES relaxation was abolished by TTX, unaffected by hexamethonium (100 microM), and enhanced by atropine (3 microM). Nicotine-LES relaxation was higher than EFS relaxation, reduced by TTX or atropine, and blocked by hexamethonium. After l-NNA, EFS elicited a strong cholinergic contraction in circular LES and EB, and nicotine elicited a small relaxation in LES and no contractile effect in EB. After l-NNA and apamin, EFS elicited a strong cholinergic contraction in LES and EB, and nicotine elicited a weak contraction amounting to 6.64 +/- 3.19 and 9.20 +/- 5.51% of that induced by EFS. EFS elicited a contraction in longitudinal strips; after l-NNA and apamin, nicotine did not induce any response. Inhibitory EMN tonically inhibit myogenic LES tone and are efficiently stimulated both by EFS and nicotinic acetylcholine receptors (nAChRs) located in somatodendritic regions and nerve terminals, releasing nitric oxide and an apamin-sensitive neurotransmitter. In contrast, although esophageal excitatory EMN are efficiently stimulated by EFS, their stimulation through nAChRs is difficult and causes weak responses, suggesting the participation of nonnicotinic mechanisms in neurotransmission to excitatory EMN in human esophagus.

Immunohistochemical characterization of putative primary afferent (sensory) myenteric neurons in human small intestine

Pseudouni- or multiaxonal Dogiel type II neurons are the intrinsic primary afferent (sensory) neurons (IPANs) in the guinea pig small intestine. Our aim was to decipher the chemical code of human myenteric type II neurons and to establish their putative vertical projections, i.e., from the myenteric plexus to the submucosa/mucosa. Additionally, we tried to distinguish them chemically from uniaxonal, dendritic type V neurons displaying, at first glance, similar shapes, i.e., smoothly contoured cell bodies with several long processes. Wholemount preparations of the myenteric plexus were immunohistochemically double or triple stained for neurofilaments (NF) and one or two of the following peptides: calbindin, calretinin (CR), calcitonin gene-related peptide (CGRP), somatostatin (SOM) and substance P (SP). In each triple stained wholemount three counts were conducted: (1) NF-positive pseudouni- or multiaxonal (type II) neurons including their reactivities for the above peptides, (2) uniaxonal or NF-negative neurons displaying coreactivities for the above peptides and (3) NF-reactive type V neurons taking into account their reactivities for the above markers. Additionally, type II neurons, which had an axon leading into (disrupted) interconnecting strands towards the submucosa were counted and somal areas of types II and V neurons were measured. The majority of myenteric type II neurons displayed coreactivities for SOM/CR (89.6%), SOM/SP (86.6%) and SP/CR (81.6%), respectively. A minority of type II neurons was positive for CGRP or calbindin. A small population with type III morphology (uniaxonal, long and slender dendrites) displayed the same coreactivities as type II neurons. In contrast, not one single type V neuron was coreactive for SOM/CR, SOM/SP or SP/CR. Out of 627 type II neurons counted in six wholemounts, 84 type II neurons displayed an axon which could be followed into disrupted interconnecting strands indicating a vertical projection pattern. Somal areas of type II neurons were twice as big as those of type V neurons (904+/-210 versus 449+/-110 microm(2)). In conclusion, most human myenteric type II neurons contain SOM, SP and CR. We suggest they are the human IPANs. Type V neurons are both morphologically and chemically distinctly different from type II neurons and may represent descending interneurons. Further studies have to decipher the type-specific chemical code of type II neurons distinguishing them also from type III neurons.

The human enteric nervous system

Decades of work in animal models have demonstrated that the enteric nervous system (ENS) plays a key role in controlling gut functions. Recent advances made it possible to extend such studies to the ENS of man in health and even in disease. Such studies have already provided new insights into the pathophysiology of inflammatory and possibly functional bowel diseases. Studies on human ENS revealed both important similarities and differences between the ENS of man and of experimental animals. Here we summarize the current state of knowledge of the electrophysiology and neurochemistry of the human ENS, including relevant reflex mediated functions in the human gut. Additionally, we review disease associated changes in human ENS properties. Finally, we highlight some research areas that hold special promise in advancing our understanding of the human ENS.

Effects of PGE2 in guinea pig colonic myenteric ganglia

PGE(2) is a proinflammatory mediator that can influence many cell types. This study was conducted to determine whether PGE(2) alters the electrical activity of distal colonic myenteric neurons, because colitis is typically associated with altered motility and changes in neural signaling may be involved. The electrical properties of intact myenteric neurons were evaluated with intracellular microelectrodes. Acute application of PGE(2) elicited a prolonged depolarization in both AH and S neurons with little effect on input resistance or electrical excitability. PGE(2) effects were suppressed by tetrodotoxin (TTX) or neurokinin (NK) receptor antagonists, indicating that PGE(2) acts directly and indirectly to depolarize colonic neurons. PGE(2)-evoked depolarization was concentration dependent (approximately 3 microM EC(50)) and was attenuated by the E prostanoid (EP)1/2 receptor antagonist, AH-6809. When preparations were maintained for 48 h in the presence of the stable PGE(2) analog PGE(2)-ethanolamide (10 microM), neurons exhibited a significant membrane depolarization and enhanced excitability. These results suggest that PGE(2) can play a role in altered motility in colitis by evoking changes in the electrical properties of myenteric neurons.

Characterization of circular muscle motor neurons of the duodenum and distal colon in the Australian brush-tailed possum

The motor innervation of the duodenum and distal colon remains uncharacterized within the same species. Our aim was to compare the projections and neurochemical properties of duodenal and distal colon circular muscle motor neurons. Circular muscle motor neurons were retrogradely traced by using a neural tracer in vitro, processed for choline acetyltransferase (ChAT) and nitric oxide synthase (NOS) immunoreactivity and then visualized by using indirect immunofluorescence. A mean of 372 +/- 64 and 156 +/- 23 neurons (mean +/- SEM) were tracer-labeled within the duodenum and colon, respectively. The ChAT+/NOS- neurons comprised 57.6 +/- 6.6% and 39.6 +/- 4.4% of all labeled cells in the duodenum and colon, respectively, and projected mainly in the oral direction. Of all labeled cells, the ChAT-/NOS+ neurons comprised 8.5 +/- 2.3% in the duodenum and 46.6 +/- 5.0% in the distal colon and projected mainly in the anal direction. Of the remainder, 20.6 +/- 5.0% and 8.2 +/- 2.4% were ChAT+/NOS+ and 13.2 +/- 0.9% and 5.6 +/- 1.4% were ChAT-/NOS- in the duodenum and distal colon, respectively. Within both regions, the distribution of the ChAT+/NOS- and ChAT-/NOS+ neurons are consistent with the ascending excitatory and descending inhibitory reflexes. The proportion of ChAT-/NOS+ neurons is greater within the colon in comparison with the duodenum. A considerable proportion of duodenal motor neurons were ChAT+/NOS+ and ChAT-/NOS-. These two classes may underlie nonperistaltic motor patterns, which predominate within the duodenum. These findings demonstrate regional differences in the innervation of intestinal circular muscle.

Neurotransmitter coding of enteric neurones in the submucous plexus is changed in non-inflamed rectum of patients with Crohn's disease

Knowledge of the neurochemical coding of submucosal neurones in the human gut is important to assess neuronal changes under pathological conditions. We therefore investigated transmitter colocalization patterns in rectal submucosal neurones in normal tissue (n=11) and in noninflamed tissue of Crohn's disease (CD) patients (n=17). Neurone-specific enolase (NSE), choline acetyltransferase (ChAT), vasoactive intestinal polypeptide (VIP), substance P (SP), nitric oxide synthase (NOS) and calcitonin gene-related peptide (CGRP) were detected immunohistochemically in whole-mount preparations from rectal biopsies. The neuronal marker NSE revealed no differences in the number of cells per ganglion (controls 5.0; CD 5.1). Four cell populations with distinct neurochemical codes were identified. The sizes of the populations ChAT/VIP (58% vs. 55%), ChAT/SP (8% vs. 8%), and ChAT/- (22% vs. 22%) were similar in control and CD. The population VIP/- was significantly increased in CD (12% vs. 2% in controls). Unlike in controls, all NOS neurones colocalized ChAT in CD. Thickened CGRP-fibres occurred in CD. We identified neurochemically distinct populations in the human submucous plexus. The increase in the VIP/- population, extensive colocalization of ChAT and NOS and hypertrophied CGRP fibres indicated adaptive changes in the enteric nervous system in noninflamed rectum of CD patients.

Retrograde tracing of enteric neuronal pathways

Neuroanatomical tracing techniques, and retrograde labelling in particular, are widely used tools for the analysis of neuronal pathways in the central and peripheral nervous system. Over the last 10 years, these techniques have been used extensively to identify enteric neuronal pathways. In combination with multiple-labelling immunohistochemistry, quantitative data about the projections and neurochemical profile of many functional classes of cells have been acquired. These data have revealed a high degree of organization of the neuronal plexuses, even though the different classes of nerve cell bodies appear to be randomly assorted in ganglia. Each class of neurone has a predictable target, length and polarity of axonal projection, a particular combination of neurochemicals in its cell body and distinctive morphological characteristics. The combination of retrograde labelling with targeted intracellular recording has made it possible to target small populations of cells that would rarely be sampled during random impalements. These neuroanatomical techniques have also been applied successfully to human tissue and are gradually unravelling the complexity of the human enteric nervous system.

Morphological and neurochemical identification of enteric neurones with mucosal projections in the human small intestine

Data on the axonal projections of enteric neurones in the human intestine are still scarce. The present study aimed to identify the morphology and neurochemical coding of enteric neurones in the human small intestine, which are involved in the innervation of the mucosa. The lipophilic neuronal tracer DiI was applied to one mucosal villus of small intestinal resection specimens. The tissue was kept in organotypic culture and subsequently processed for immunohistochemistry. Neurones labelled from the mucosa were located in all ganglionated nerve networks, including the myenteric plexus. In all plexuses, at least five neurochemical types of neurones could be observed, i.e. SOM-IR neurones, SP-IR neurones, SOM/SP-IR neurones, VIP-IR neurones and neurones lacking immunoreactivity for any of these markers. Most of the DiI-labelled neurones were multidendritic; a minority of neurones could be identified as Dogiel type II cells, suggesting the existence of a subgroup of primary afferent neurones in the DiI-filled cell population. The ratio of labelled multidendritic neurones (assumed to be secretomotor) to labelled Dogiel type II neurones (assumed to be primary afferent) in the myenteric plexus is higher in large mammals (pig and human) than in small mammals (guinea pig). This might point to the existence of a different topographical distribution of subsets of primary afferent neurones and/or topographically distinct intrinsic mucosal reflex circuits in large mammals, including humans.

Role of nitric oxide- and vasoactive intestinal polypeptide-containing neurones in human gastric fundus strip relaxations

The morphological pattern and motor correlates of nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) innervation in the human isolated gastric fundus was explored. By using the nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-diaphorase and specific rabbit polyclonal NO-synthase (NOS) and VIP antisera, NOS- and VIP-containing varicose nerve fibres were identified throughout the muscle layer or wrapping ganglion cell bodies of the myenteric plexus. NOS-immunoreactive (IR) neural cell bodies were more abundant than those positive for VIP-IR. The majority of myenteric neurones containing VIP coexpressed NADPH-diaphorase. Electrical stimulation of fundus strips caused frequency-dependent NANC relaxations. N(G)-nitro-L-arginine (L-NOARG: 300 microM) enhanced the basal tone, abolished relaxations to 0.3 - 3 Hz (5 s) and those to 1 Hz (5 min), markedly reduced ( approximately 50%) those elicited by 10 - 50 Hz, and unmasked or potentiated excitatory cholinergic responses at frequencies > or =1 Hz. L-NOARG-resistant relaxations were virtually abolished by VIP (100 nM) desensitization at all frequencies. Relaxations to graded low mechanical distension (< or =1 g) were insensitive to tetrodotoxin (TTX: 1 microM) and L-NOARG (300 microM), while those to higher distensions (2 g) were slightly inhibited by both agents to the same extent ( approximately 25%). In the human gastric fundus, NOS- and VIP immunoreactivities are colocalized in the majority of myenteric neurones. NO and VIP mediate electrically evoked relaxations: low frequency stimulation, irrespective of the duration, caused NO release only, whereas shortlasting stimulation at high frequencies induced NO and VIP release. Relaxations to graded mechanical distension were mostly due to passive viscoelastic properties, with a slight NO-mediated neurogenic component at 2 g distension. The difference between NO and VIP release suggests that in human fundus accommodation is initiated by NO. British Journal of Pharmacology (2000) 129, 12 - 20

Projections of nitric oxide synthase and vasoactive intestinal polypeptide-reactive submucosal neurons in the human colon

BACKGROUND

The submucosal plexus is important in the control of secretomotor and motor function of the intestine. Our aim was to describe the projections of submucosal neurons to the mucosa within the submucosal plexus and to the circular muscle of human colon and to determine whether submucosal neurons that projected to different layers were located at different levels of the submucosa.

METHODS

A retrogradely transported fluorescent dye was applied to the mucosa, submucosa or circular muscle layer of human colon which was then maintained in organotypic culture for 5 days. The submucosa was then dissected into two preparations, one containing the inner layer of the submucosal plexus and the other containing both the intermediate and outer layers. The dissected preparations were labelled with antibodies to nitric oxide synthase (NOS) or vasoactive intestinal peptide (VIP).

RESULTS

Submucosal neurons projected to the mucosa, submucosa and circular muscle layers for mean distances of 3.7, 3.0 and 4.3 mm, respectively. Ninety-seven per cent of submucosal neurons labelled from the circular muscle were located in the outer or the intermediate layers, while 51% of those projecting to the mucosa were in inner layer and 49% in the intermediate/outer layers of the submucosal plexus. Eleven per cent of submucosal neurons projecting to the circular muscle were immunoreactive for NOS and 12% were immunoreactive for VIP. Forty-five per cent of those projecting within the submucosa were immunoreactive for VIP and 38% of those projecting to the mucosa were immunoreactive for VIP.

CONCLUSIONS

Submucosal neurons in the human colon innervate the mucosa, circular muscle and submucosa and different functional classes of neurons are located in different layers of the submucosal plexus.

Differential inflammatory modulation of canine ileal longitudinal and circular muscle cells

The aim of this study was to identify the subtypes of muscarinic receptors that mediate in vivo and in vitro canine ileal longitudinal muscle contractions and whether their role is modulated by inflammation. Previous studies have reported that circular muscle contractions are suppressed in ileal inflammation induced by mucosal exposure to ethanol and acetic acid. We found that inflammation had no significant effect on in vivo and in vitro spontaneous or muscarinic receptor-mediated contractions of the longitudinal muscle. The longitudinal muscle contractions were mediated primarily by the M(3) receptor subtype. However, the IC(50) of the M(2) receptor antagonist methoctramine was only 10 times greater than that of the M(3) receptor antagonist 4-DAMP in the longitudinal muscle, whereas it was 224 times greater in the circular muscle. M(2) receptor-coupled decrease of intracellular cAMP occurred in the longitudinal but not in the circular muscle from the normal ileum. Inflammation did not alter this coupling in the longitudinal muscle but established it in the circular muscle. In conclusion, M(2) receptors may play a greater role in the mediation of longitudinal muscle contractions than circular muscle contractions. Inflammation does not alter the contractility or the relative role of muscarinic receptor subtypes in longitudinal muscle cells. However, it modulates the M(2) receptor coupling to adenylate cyclase in the circular muscle.

Identification of motor neurons to the circular muscle of the guinea pig gastric corpus

The projections of enteric neurons to the circular muscle of the guinea pig gastric corpus were investigated systematically by using the retrogradely transported fluorescent carbocyanine dye 1,1'-didodecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI), applied to the muscle layer or myenteric plexus in vitro. DiI-labeled motor neuron cell bodies were located up to 6.3 mm aboral, 17 mm oral, and up to 20 mm circumferential to the DiI application site. Labeled nerve fibers ran for long distances from the DiI application site toward the greater and lesser curvatures, where they coursed parallel to the bundles of the "gastric sling" muscle. The majority of labeled cells were located toward the lesser curvature of the stomach. Nerve cell bodies that were aboral to the DiI application site were usually small, immunoreactive for choline acetyltransferase, and, thus, were likely to be excitatory motor neurons. Neurons that were located orally were larger, fewer in number, and immunoreactive for nitric oxide synthase and, thus, were likely to be inhibitory motor neurons. Application of DiI directly to the myenteric plexus filled neurons up to 15 mm aborally and up to 21 mm orally but labeled few neurons circumferentially. All nerve cells that were filled from either the circular muscle or the myenteric plexus had Dogiel type I morphological features. These results demonstrate a clear polarity of projection of inhibitory and excitatory motor neurons and a functionally continuous innervation of the circular and gastric sling muscle layers. Nonmotor neurons in the myenteric plexus were demonstrated, but neurons with Dogiel type II morphological features are apparently absent.

Characterisation of the adventitial rectal ganglia in the male rat by their immunohistochemical features and projections

In recent years, considerable progress has been made in characterising the neural circuitry of the pelvic plexus, particularly in the male rat. However, the small ganglia on the adventitial surface of the rectum remain largely unstudied. We have used immunohistochemistry and retrograde tracing techniques to determine the content and projections of these neurons. The adventitial ganglia contain 600-1,000 neurons. All of these are immunoreactive for choline acetyltransferase, 44% are immunoreactive for calbindin, and 35% are immunoreactive for vasoactive intestinal peptide. Very few (1-5%) adventitial neurons contain tyrosine hydroxylase or neuropeptide Y. In contrast, most adventitial neurons are surrounded by varicose axons that do contain tyrosine hydroxylase or neuropeptide Y. Retrograde tracing studies showed that the primary targets of adventitial neurons within the bowel are the internal anal sphincter and the circular muscle directly adjacent to the sphincter. However, more adventitial neurons project out of the gut wall than to targets within the bowel. These are most likely to be viscerofugal and rectospinal neurons. Combining the immunohistochemical and tracing observations, these studies suggest that the rat adventitial ganglia do not represent an additional source of pelvic (autonomic postganglionic) neurons but, instead, that they are comprised primarily of viscerofugal and rectospinal neurons. This is very different from the adventitial rectal ganglia of the cat, which represent merely an extension of the pelvic plexus.

Projections of pelvic autonomic neurons within the lower bowel of the male rat: an anterograde labelling study

The tissues of the large intestine which receive an innervation by neurons of the major pelvic ganglia were identified following in vivo and in vitro anterograde labelling with the lipophilic tracer 1,1'didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate in the male rat. The primary target in the gut of major pelvic ganglion neurons is the myenteric plexus of the distal colon and the rectum. The serosal ganglia, on the surface of the most distal region of the rectum and the circular muscle of the distal colon and rectum were less densely innervated. The pelvic ganglia do not innervate the longitudinal muscle, submucosal blood vessels, submucosal plexus, or mucosa. The pelvic supply reaches the bowel via two groups of rectal nerves and branches of the penile nerves. All of these connections also carry the axons of viscerofugal neurons from the bowel, some of which have terminal axons in the major pelvic ganglia. Finally, the different nerves supplied different targets. In particular, while the rectal nerves carried pelvic axons supplying the myenteric plexus, circular muscle, and serosal ganglia, the penile nerves only innervated the serosal ganglia. In addition, the two groups of rectal nerves innervated slightly different regions of the bowel and provided different projection patterns. However, successful in vivo labelling was achieved in only 6/12 animals and while all in vitro experiments resulted in successful labelling, it was clear that only a proportion of pelvic projections in any given nerve were labelled. These studies have shown that the major pelvic ganglia are primarily involved in the control of motility, but not of vascular and secretomotor functions. Thus pelvic neurons do not innervate the same range of target tissues within the bowel as the prevertebral ganglia. This study has also shown that the different pathways to the gut from the major pelvic ganglia innervate different tissues, suggesting that the autonomic innervation of the gut is not homogeneous along its length.

The denervated stomach as an esophageal substitute is a contractile organ

OBJECTIVE

To determine whether the denervated stomach as an esophageal substitute is an inert conduit or a contractile organ.

SUMMARY BACKGROUND DATA

The motor response of gastric transplants to deglutition suggests that the stomach pulled up to the neck acts as an inert organ.

METHODS

The gastric motility of 11 healthy volunteers and 33 patients having either a gastric tube (GT) (n = 10) or their whole stomach (WS) (n = 23) as esophageal replacement was studied with perfused catheters during the fasting state, after a meal, and after intravenous administration of erythromycin lactobionate. A motility index was established for each period of recording by dividing the sum of the areas under the curves of all contractions of >9 mmHg by the time of recording.

RESULTS

Over years, the denervated stomach recovers more and more motor activity, even displaying a real phase 3 motor pattern in 6 of the 10 WS patients and 1 of the 7 GT patients with >3 years of follow-up. Erythromycin lactobionate generates a phase 3-like motor pattern regardless of the length of follow-up. Extrinsic denervation of the whole stomach does not significantly modify the fasting motility index established >3 years after surgery (+17% on average, p > 0.05), but it reduces that in the fed period by an average of 62% (p = 0.0016). Tubulization of the denervated whole stomach lowers the fasting motility index by an average of 60% (p = 0.0248) and further impairs that in the fed period by an average of 67% (p = 0.0388).

CONCLUSIONS

The denervated stomach as an esophageal substitute is a contractile organ that may even generate complete migrating motor complexes. Motor recovery is better in the fasting than in the fed period, and it is more marked in WS patients than in GT patients.

Projections of chemically identified myenteric neurons of the small and large intestine of the mouse

The projections of different subpopulations of myenteric neurons in the mouse small and large intestine were examined by combining immunohistological techniques with myotomy and myectomy operations. The myotomies were used to examine the polarity of neurons projecting within the myenteric plexus and showed that neurons containing immunoreactivity for nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), calbindin and 5-HT projected anally, while neurons with substance P (SP)-immunoreactivity projected orally, in both the small and large intestine. Neurons containing neuropeptide Y (NPY)- and calretinin-immunoreactivity projected locally. In the large intestine, GABA-immunoreactive neurons projected both orally and anally, with more axons tending to project anally. Myectomy operations revealed that circular muscle motor neurons containing NOS/VIP/ +/-NPY and calretinin neurons projected anally both in the small and large intestine, while SP-immunoreactive circular muscle motor neurons projected orally. In the large intestine, GABA-IR circular muscle motor neurons projected both orally and anally. This study showed that although some neurons, such as the NOS/VP inhibitory motor neurons and interneurons, SP excitatory motor neurons and 5-HT interneurons had similar projections to those in other species, the projections of other chemical classes of neurons in the mouse intestine differed from those reported in other species.

In situ identification and visualization of neurons that mediate enteric and enteropancreatic reflexes

To identify neurons participating in enteric and enteropancreatic reflexes, we validated the use of the activity-dependent markers FM1-43 and FM2-10 as "on-line" probes for the visualization of activated guinea pig enteric and pancreatic neurons. FM1-43 or FM2-10 labeling of neuronal perikarya and processes was induced by KCl (70 mM), veratridine (1.0 microM), intracellular injection of depolarizing current pulses, stimulation of afferent inputs, evoking reflexes (by inflating an intraluminal balloon, blowing puffs of N2 at, or applying glucose to, the villous surface of the duodenum), or injury; labeling was prevented by tetrodotoxin (0.5 microM). Intracellular recording and injection of Neurobiotin confirmed that FM1-43 labeled neurons that spike, but not those that exhibit only fast excitatory postsynaptic potentials. Perikarya did not label if axonal transport was blocked by colchicine. When pulses of N2 or glucose were directed at duodenal villi in vitro, labeling by FM1-43 or FM2-10 was observed in myenteric and pancreatic neurons, as well as in subsets of cells in pancreatic islets and intestinal crypts. Hexamethonium blocked the spread of label via nicotinic synapses and thus enabled primary afferent neurons to be located. Balloon distension elicited hexamethonium-resistant labeling of epithelial cells, interstitial cells, and Dogiel type II neurons in each plexus; however, in preparations stimulated with pulses of N2 or glucose, hexamethonium-resistant labeling of neurons occurred only in the submucosal plexus and not in myenteric ganglia. These observations suggest that primary afferent neurons responsible for mucosal pressure- or glucose-induced enteric and enteropancreatic reflexes are submucosal, whereas myenteric afferent neurons become activated only when the wall of the bowel is distended. The data are compatible with the possibility that primary afferent neurons are activated by a signaling molecule released from intestinal epithelial cells.