Nitric oxide synthase immunoreactivity in the enteric nervous system of the developing human digestive tract

We have investigated indirectly the presence of nitric oxide in the enteric nervous system of the digestive tract of human fetuses and newborns by nitric oxide synthase (NOS) immunocytochemistry and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry. In the stomach, NOS immunoactivity was confined to the myenteric plexus and nerve fibres in the outer smooth musculature; few immunoreactive nerve cell bodies were found in ganglia of the outer submucous plexus. In the pyloric region, a few nitrergic perikarya were seen in the inner submucous plexus and some immunoreactive fibers were found in the muscularis mucosae. In the small intestine, nitrergic neurons clustered just underneath or above the topographical plane formed by the primary nerve strands of the myenteric plexus up to the 26th week of gestation, after which stage, they occurred throughout the ganglia. Many of their processes contributed to the dense fine-meshed tertiary nerve network of the myenteric plexus and the circular smooth muscle layer. NOS-immunoreactive fibres directed to the circular smooth muscle layer originated from a few NOS-containing perikarya located in the outer submucous plexus. In the colon, caecum and rectum, labelled nerve cells and fibres were numerous in the myenteric plexus; they were also found in the outer submucous plexus. The circular muscle layer had a much denser NOS-immunoreactive innervation than the longitudinally oriented taenia. The marked morphological differences observed between nitrergic neurons within the developing human gastrointestinal tract, together with the typical innervation pattern in the ganglionic and aganglionic nerve networks, support the existence of distinct subpopulations of NOS-containing enterice neurons acting as interneurons or (inhibitory) motor neurons.

Structural organization and neuropeptide distribution in the mammalian enteric nervous system, with special attention to those components involved in mucosal reflexes

Gastrointestinal events such as peristalsis and secretion/absorption processes are influenced by the enteric nervous system, which is capable of acting largely independently from other parts of the nervous system. Several approaches have been used to further our understanding of the underlying mechanisms of specific enteric microcircuits. Apart from pharmacological and physiological studies, the deciphering of the chemical coding of distinct morphological and functional enteric neuron classes, together with a detailed analysis of their projections by the application of immunocytochemistry, of tracing, and of denervation techniques, have substantially contributed to our knowledge. In view of existing interspecies and regional differences, it is of major importance to expand our knowledge of the enteric nervous system in mammals other than the guinea-pig, the most commonly used experimental animal in this research area. This will increase our chances of finding a valid model, from which well-founded extrapolations can be made regarding the precise function of distinct enteric neuron types regulating motility and ion transport in the human gastrointestinal tract.

Pulmonary intraepithelial vagal nodose afferent nerve terminals are confined to neuroepithelial bodies: an anterograde tracing and confocal microscopy study in adult rats

Our present understanding of the morphology of neuroepithelial bodies (NEBs) in mammalian lungs is comprehensive. Several hypotheses have been put forward regarding their function but none has been proven conclusively. Microscopic data on the innervation that appears to affect the reaction of NEBs to stimuli have given rise to conflicting interpretations. The aim of this study has been to check the validity of the hypothesis that pulmonary NEBs receive an extensive vagal sensory innervation. The fluorescent neuronal tracer DiI was injected into the vagal sensory nodose ganglion and NEBs were visualized in toto by using immunocytochemistry and confocal microscopy on 100-micrometer-thick frozen sections of the lungs of adult rats. The most striking finding was the extensive intraepithelial terminal arborizations of DiI-labelled vagal afferents in intrapulmonary airways, apparently always co-appearing with calcitonin gene-related peptide (CGRP)-immunoreactive NEBs. Not all NEBs received a traced nerve fibre. Intrapulmonary CGRP-containing nerve fibres, including those innervating NEBs, always appeared to belong to a nerve fibre population different from the DiI-traced fibres and hence did not arise from the nodose ganglion. Therefore, at least some of the pulmonary NEBs in adult rats are supplied with sensory nerve fibres that originate from the vagal nodose ganglion and form beaded ramifications between the NEB cells, thus providing support for the hypothesis of a receptor function for NEBs.

Distinct distribution of CGRP-, enkephalin-, galanin-, neuromedin U-, neuropeptide Y-, somatostatin-, substance P-, VIP- and serotonin-containing neurons in the two submucosal ganglionic neural networks of the porcine small intestine

In addition to differences between the two submucosal ganglionic neural networks, i.e., the plexus submucosus externus (Schabadasch) and the plexus submucosus internus (Meissner), with respect to the occurrence and distribution of serotonin as neurotransmitter, immunocytochemistry also revealed a distinct distribution for various neuropeptides in these two plexuses. Immunoreactivity for galanin, vasoactive intestinal polypeptide, calcitonin gene-related peptide, substance P, neuromedin U, enkephalin, somatostatin and neuropeptide Y was found in varicose and non-varicose nerve fibres of both submucosal ganglionic plexuses, albeit with a distinct distributional pattern. The difference in neurotransmitter and/or neuromodulator content between both neural networks became even more obvious when attention was focussed on the immunoreactivity of the nerve cell bodies for these substances. Indeed, neuropeptide Y, enkephalin- and somatostatin-immunoreactive neuronal perikarya as well as serotonergic neuronal cell bodies appear solely in the plexus submucosus externus. Neuromedin U-immunoreactive perikarya, mostly coexisting with substance P, are observed in large numbers in the plexus submucosus internus, whilst they are rare in the plexus submucosus externus. Double-labelling immunostaining for substance P with CGRP and galanin revealed a different coexistence pattern for the two submucosal ganglionic plexuses. The differing chemical content of the neuronal populations supports the hypothesis that the existence of the two submucosal ganglionic plexuses, present in most large mammals including man, not only reflects a morphological difference but also points to differentiated functions.

Neuron-specific enolase and S-100 protein immunohistochemistry for defining the structure and topographical relationship of the different enteric nerve plexuses in the small intestine of the pig

The morphological and topographical features of the intramural enteric nervous system in the small intestine of the pig has been studied on whole mounts by means of neuron-specific enolase (NSE) and S-100 protein immunohistochemistry. A clear visualization of the myenteric plexus allows the recognition of its characteristic morphology, including the thin tertiary plexus coursing within the smooth muscle layers. In the tela submucosa two ganglionated plexuses, each with its own specific characteristics, can clearly be demonstrated: (1) the plexus submucosus externus (Schabadasch) located near the inner surface of the circular muscle layer at the abluminal side of the submucosal vascular arcades, and (2) the plexus submucosus internus (Meissner) close to the outer surface of the lamina muscularis mucosae at the luminal side of the submucosal vascular arcades. Due to the possibility to trace clearly the perivascular plexuses of these vascular arcades by use of immunohistochemical techniques with antibodies to NSE and S-100 protein, the two submucosal nerve plexuses can be demonstrated with exceptional clarity. This is the first report of an investigation of the intramural nerve plexuses of the small intestine of the pig using the NSE and S-100 immunostaining methods, which is sufficiently detailed to substantiate the characteristic topography and structure of the two submucosal plexuses and their relation to the smooth muscle layers and perivascular plexuses. The level of NSE immunoreactivity for enteric neurons displays great variation, a substantial proportion of the type-II neurons appearing strongly stained. Although little is known of the specific function of these enzymes, proposals are discussed.

Distribution pattern, neurochemical features and projections of nitrergic neurons in the pig small intestine

The presence and topographical distribution of nitrergic neurons in the enteric nervous system (ENS) of the pig small intestine have been investigated by means of nitric oxide synthase (NOS) immunocytochemistry and nicotinamide dinucleotide phosphate diaphorase (NADPHd) histochemistry. Both techniques yielded similar results, thus confirming that within the pig ENS the neuronal isoform of NOS corresponds to NADPHd. Intrinsic nitrergic neurons were not confined to the myenteric plexus; considerable numbers were also present in the outer submucous plexus. In the inner submucous plexus, NOS immunoreactivity or NADPHd staining was restricted to a few nerve fibres and nerve cell bodies. The nitrergic neurons displayed a wide variety in size and shape, but could all be characterized as being multidendritic uniaxonal. Nerve lesion experiments showed that the majority of the myenteric nitrergic neurons project in an anal direction. Evidence is at hand to show that a substantial proportion of these neurons contribute to the dense nitrergic innervation of the tertiary plexus and the circular smooth muscle layer. Some of the nitrergic neurons of the outer submucous plexus were equally found to send their axons towards the circular muscle layer. In some of the nitrergic enteric neurons, VIP, neuropeptide Y, galanin or protein 10 occurred colocalized, but not calbindin or serotonin. The present findings provide morphological evidence for the presence of NOS in a proportion of the enteric neurons in the small intestine of a large omnivorous mammal, i.e. the pig. The topographical features of the staining patterns of NOS and NADPHd are in accord with the results of neuropharmacological studies and argue for the existence of distinct nitrergic subpopulations acting either as interneurons or as motor neurons.

Distribution and morphological features of nitrergic neurons in the porcine large intestine

The distribution of nitric oxide synthase (NOS), an enzyme involved in the synthesis of the presumed non-adrenergic noncholinergic inhibitory neurotransmitter nitric oxide (NO), was demonstrated in the enteric nervous system of the porcine caecum, colon and rectum. Techniques used were NOS-immunocytochemistry and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd)-histochemistry. Throughout the entire large intestine, NOS-immunoreactive (IR) and NADPHd-positive neurons were abundant in the myenteric and outer submucous plexus. In the inner submucous plexus, only a small number of positive neurons were found in the caecum and colon, while a moderate number was observed in the rectum. The nitrergic neurons in the porcine enteric nerve plexuses were of a range of sizes and shapes, with a small proportion showing immunostaining for vasoactive intestinal polypeptide. Varicose and non-varicose NOS-IR and NADPHd-positive nerve fibres were present in the ganglia and connecting strands of all three plexuses. Nerve fibres were also numerous in the circular muscle layer, scarce in the longitudinal muscle coat and negligible in the mucosal region. The abundance of NOS/NADPHd in the intrinsic innervation of the caecum, colon and rectum of the pig implicates NO as an important neuronal messenger in these regions of the gastrointestinal tract.

Calcitonin gene-related peptide in morphologically well-defined type II neurons of the enteric nervous system in the porcine small intestine

The morphological classification of the different neuronal cell types is generally accepted and expanded by us; nevertheless, immunohistochemically and electrophysiologically the existence of clear-cut categories of enteric neurons is frequently questioned. The immunohistochemical results demonstrated in this study, however, provide the first direct link between a morphological type of enteric neuron and an immunohistochemical staining for a distinct peptide. Evidence demonstrates that calcitonin gene-related peptide occurs in only one morphologically defined type of neuron, viz., in type II neurons, and can therefore be regarded as a 'marker peptide' for type II neurons. Hence, the present immunohistochemical findings illustrate the validity of the morphological classification of the enteric neurons.

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.

Pulmonary macrophages in birds (barn owl, Tyto tyto alba), domestic fowl (Gallus gallus f. domestica), quail (Coturnix coturnix), and pigeons (Columbia livia)

BACKGROUND

Birds have a limited number of resident macrophages in the normal steady-state respiratory tract. The discovery of phagocytes in lavages of lung from birds contrasts with findings that phagocytes are seldom seen in investigations in situ. An electron microscopic study was performed in the respiratory units, the parabronchi, and air capillaries in particular in several adult bird species to localize the seat of respiratory macrophages.

METHODS

Lung tissue of barn owl, domestic fowl, quail, and town and homing pigeons was subjected to standard processing for light and electron microscopy after immersion fixation, intratracheal instillation, and intravascular perfusion.

RESULTS

Clusters of macrophages were predominantly housed in the loose connective tissue at the floor of atria at the entrance to the infundibula and gas-exchange tissue proper. Scattered solitary phagocytes were also found in connective tissue of air sacs, interatrial septa, and adventitia of inter- and intraparabronchial arteries and veins and in peribronchial lymphoid tissue. Phagocytized foreign particulate material mostly consists of hard, dense, crystalline formations surrounded by a limiting membrane. The transport of small airborne particles occurs via the squamous atrial epithelium to the underlying macrophages. The macrophages are often accompanied by mast cells.

CONCLUSIONS

The present results demonstrate that avian respiratory macrophages are predominantly located in atrial connective tissue compartments and do not seem to migrate to the airway surfaces.

Calcitonin gene-related peptide-like immunoreactivity in the human small intestine

Calcitonin-gene-related-peptide (CGRP)-like immunoreactivity was localized in nerve fibres, neuronal somata and in mucosal endocrine cells of the human small intestine. Immunoreactive enteric neurons were more numerous in the submucous plexuses than in the myenteric plexus. Morphologically, they predominantly had the appearance of type II neurons. The majority of the CGRP-like immunoreactive nerve fibres ran within the ganglionic nerve plexuses. Only a small proportion could be observed in the lamina propria, the lamina muscularis mucosae, or the circular and longitudinal outer smooth muscle layer. These findings suggest that within the wall of the human small intestine neuronal CGRP of either extrinsic or intrinsic origin exerts its effect chiefly on other enteric neurons, and might be indirectly involved in the regulatory functions of the human small intestine.

An electron microscopic study of the parabronchial epithelium in the mature lung of four bird species

BACKGROUND

No integrated comprehensive description of the ultrastructure of the parabronchial epithelium is available. The origin, discharge, and occurrence of the trilaminar substance have not yet been sufficiently studied. Therefore, the main objectives were to classify the cell types of the parabronchial epithelium and to describe their role in manufacturing the trilaminar substance.

METHODS

Lung tissue of mature quail, domestic fowl, town pigeon, homing pigeon, and barn owl was subjected to standard processing for transmission electron microscopy, both after intratracheal inflation and intravascular perfusion.

RESULTS

The atrial epithelium is constituted by granular and squamous atrial cells. Granular cells (1) are confined to the atrial wall; they produce and discharge osmiophilic lamellar bodies. Squamous atrial cells (2) manufacture and discharge a trilaminar substance in sheets sandwiched between the long microvilli emerging from the apical cytoplasm. Their attenuating cell outgrowths overlap granular cells. At the bases of atria, they pass as intermediate squamous atrial cells to the infundibula, contacting the extensions of squamous respiratory cells. The squamous atrial cells undergo distinct structural variations depending on age and environment. Squamous respiratory cells (3) (cellulae squamosae) continuously line the air capillaries and neighboring infundibula. They constitute the epithelial compartment of the blood-gas barrier. The cell bodies extend long, very thin cell outgrowths. The apical surface is smooth and the basal part is invested with a very thin basement membrane. The trilaminar substance originates from granular and agranular endoplasmic reticulum in the form of convoluted profiles which are discharged as an acellular lining layer on the air surface of squamous respiratory cells.

CONCLUSIONS

Granular cells are analogous to the type II cells of mammalian pulmonary alveolus. Squamous atrial and respiratory cells, of a common embryonic origin, do not meet any counterpart in epithelial cell populations of lung terminal airways in vertebrates. The specific trilaminar substance--lipoproteinaceous in nature--is a constant compound of atria and air capillaries.

A correlative light-, fluorescence- and electron-microscopic study of neuroepithelial bodies in the lung of the red-eared turtle, Pseudemys scripta elegans

Neuroepithelial bodies (NEB) were identified for the first time in the respiratory tract of a reptile by the use of combined morphological and histochemical methods. In the red-eared turtle, Pseudemys scripta elegans, NEB were found within the trabecular epithelium of the respiratory tract, mainly in the branching regions of the trabeculae. An intracellular formaldehyde-induced fluorescent compound was identified as 5-hydroxytryptamine (5-HT) by means of microspectrofluorometry. Subsequent histochemical staining of the same fluorescent sections showed the 5-HT-containing cells to be argentaffin. In electron micrographs cell clusters characterized by the presence of distinctive cytoplasmic, membrane-bounded dense granules (+/- 100 nm) were observed, correlating with the distribution of the yellow-fluorescent epithelial cells. The granules of the NEB are positive when the argentaffin technique is performed directly on ultrathin sections. Cells of the NEB extend into the lumen of the airway via apical microvilli and a single modified cilium displaying a 9 X 2 + 0 or 8 X 2 + 2 microtubular pattern. Unmyelinated axons containing mostly small, clear vesicles were seen in close association with NEB cells, often forming synaptic junctions. Occasionally, axons containing a few small dense-cored vesicles were found. The relationship between NEB cells and capillaries, the images of emiocytotic granule release and the occurrence of synaptic contacts between axons and granule-containing cells are indicative of endocrine secretion. These features and the presence of intracytoplasmic granules containing 5-HT may justify the inclusion of NEB-cells of the turtle lung into the diffuse neuroendocrine system. Furthermore, structurally these cells appear to represent sensory elements capable of an intrapulmonary receptor-secretory function.

Differences in the distribution and chemical coding between neurons in the inferior mesenteric ganglion supplying the colon and rectum in the pig

The distribution and chemical coding of neurons in the porcine left and right inferior mesenteric ganglion projecting to the ascending colon and rectum have been investigated by using combined retrograde tracing and double-labelling immunohistochemistry. The ganglion contained many neurons supplying both gut regions. The colon-projecting neurons (CPN) occurred exclusively in the cranial part of the ganglia where they formed a large cluster distributed along the dorso-lateral ganglionic border and a smaller cluster located close to the caudal colonic nerve output. The rectum-projecting neurons (RPN) formed a long stripe along the entire length of the lateral ganglionic border and, within the right ganglion only, a small cluster located close to the caudal colonic nerve output. Immunohistochemistry revealed that the vast majority of the CPN and RPN were noradrenergic (tyrosine-hydroxylase-positive). Many noradrenergic neurons supplying the colon contained somatostatin or, less frequently, neuropeptide Y. In contrast, a significant subpopulation of the noradrenergic RPN expressed neuropeptide Y, whereas only a small proportion contained somatostatin. A small number of the non-adrenergic RPN were cholinergic (choline-acetyltransferase-positive) and a much larger subpopulation of the nerve cells supplying both the colon and rectum were non-adrenergic and non-cholinergic. Many cholinergic neurons contained neuropeptide Y. The non-adrenergic non-cholinergic neurons expressed mostly somatostatin or neuropeptide Y and some of those projecting to the rectum contained nitric oxide synthase, galanin or vasoactive intestinal polypeptide. Many of both the CPN and RPN were supplied with varicose nerve fibres exhibiting immunoreactivity against Leu5-enkephalin, somatostatin, choline-acetyltransferase, vasoactive intestinal polypeptide or nitric oxide synthase The somatotopic and neurochemical organization of this relatively large population of differently coded inferior mesenteric ganglion neurons projecting to the large bowel indicates that these cells are probably involved in intestino-intestinal reflexes controlling peristaltic and secretory activities.

Distribution and immunohistochemical characteristics of neurons in the porcine caudal mesenteric ganglion projecting to the vas deferens and seminal vesicle

Combined retrograde tracing (using fluorescent tracer Fast Blue) and double-labelling immunofluorescence were used to study the distribution and immunohistochemical characteristics of neurons in the porcine caudal mesenteric ganglion projecting to the vas deferens and seminal vesicle. The distribution and immunohistochemical properties of neurons projecting to both organs were similar. As revealed by retrograde tracing, Fast Blue-positive neurons were located within the left and right ganglia, with a distinct predominance in the ipsilateral one. In the ipsilateral ganglion, the majority of the neurons were located caudally, along the dorso-lateral ganglionic border, suggesting a somatotopic organization of the ganglion. Immunohistochemistry revealed four populations of retrogradely labelled neurons (from the largest to the smaller one): tyrosine hydroxylase-positive/neuropeptide Y-negative (TH+/NPY-), TH+/NPY+, TH-/NPY-, TH-/NPY+. With respect to their surrounding nerve fibres, two subpopulations of the dye-labelled neurons could be distinguished. The small one consisted of solitary neurons receiving a strong calcitonin gene-related peptide- and Leu5-enkephalin-, and a less intense vasoactive intestinal peptide-immunoreactive innervation. The remaining neurons were poorly supplied by singular nerve fibres containing some of the investigated peptides. We conclude that the caudal mesenteric ganglion should be considered as a prominent source of adrenergic and/or NPY-positive innervation for the porcine male reproductive tract.

Occurrence, distribution and neurochemical features of small intestinal neurons projecting to the cranial mesenteric ganglion in the pig

The small intestine of the pig has been investigated for its topographical distribution of enteric neurons projecting to the cranial mesenteric ganglion, by using Fast Blue or Fluorogold as a retrogradely transported neuronal tracer. Contrary to the situation in small laboratory animals such as rat and guinea-pig, the intestinofugally projecting neurons in the porcine small intestine were not restricted to the myenteric plexus, but were observed in greater numbers in ganglia of the outer submucous plexus. The inner submucous plexus was devoid of labelled neurons. Retrogradely labelled neurons were mostly found, either singly or in small aggregates, in ganglia located within a narrow border on either side of the mesenteric attachment. For both nerve networks, their number increased from duodenum to ileum. All the retrogradely labelled neurons exhibited a multidendritic uniaxonal appearance. Some of them displayed type-III morphology and stained for serotonin. This study indicates that, in the pig, not only the myenteric plexus but also one submucous nerve network is involved in the afferent component of intestino-sympathico-intestinal reflex pathways. The finding that some of the morphologically defined type-III neurons participate in these reflexes is in accord with the earlier proposal that type-III neurons are supposed to fulfill an interneuronal role, whether intra- or extramurally.

Electrophysiological features of morphological Dogiel type II neurons in the myenteric plexus of pig small intestine

By intracellular recording, 99 myenteric neurons with Dogiel type II morphology were electrophysiologically characterized in the porcine ileum and further subdivided into three groups based on their different types of afterhyperpolarization (AHP). In response to a depolarizing current injection, a fast AHP (fAHP; duration 34 +/- 11 ms; amplitude -11 +/- 6 mV; mean +/- SD) immediately followed every action potential in all neurons. In 32% of the neurons, this fAHP was the sole type of hyperpolarization recorded. Statistical analysis revealed the presence of two neuronal subpopulations that displayed either a long-lasting medium AHP (mAHP; duration after a single spike 773 +/- 753 ms; 51% of neurons) or a slow AHP (sAHP; 4, 205 +/- 1,483 ms; 17%). Slow AHP neurons also differed from mAHP neurons in the delayed onset of the AHP (mAHP 0 ms; sAHP 100-200 ms), as well as in maximum amplitude values and in the time to reach this amplitude (t(max); 148 +/- 11 ms vs. 628 +/- 108 ms). Medium AHP neurons further differed from the sAHP neurons in the occurrence of the AHP following subthreshold current injection and in their resting membrane potential (mAHP, -53 +/- 8 mV; sAHP, -62 +/- 10 mV). Medium AHP and sAHP behaved similarly in that a higher number of spikes increased their amplitude and duration, but not t(max). The majority of neurons fired multiple spikes (up to 25) in response to a 500-ms current injection (81/99) and showed a clear TTX-resistant shoulder on the repolarizing phase of the action potential (77/99), irrespective of the presence of sAHP or mAHP. These results demonstrate that the porcine Dogiel type II neurons differ in various essential electrophysiological properties from their morphological counterparts in the guinea pig ileal myenteric plexus. The most striking interspecies differences were the low occurrence of sAHP (17% vs. 80-90% in guinea pig) with relatively small amplitude (-5 vs. -20 mV), the high occurrence of mAHPs (unusual in guinea pig) and the ability to fire long spike trains (up to 25 spikes vs. 1-3 in guinea pig). In fact, Dogiel type II neurons in porcine ileum combine distinct electrophysiological features considered typical of either S-type or sAHP-type neurons in guinea pig. It can therefore be concluded that in spite of a similar morphology, Dogiel type II neurons do not behave electrophysiologically in a universal way in large and small mammals.

Nitric oxide synthase-containing neurons in the pig large intestine: topography, morphology, and viscerofugal projections

The distribution of neurons that are capable of synthesizing nitric oxide (NO) has been demonstrated in the porcine large intestine by means of NO synthase (NOS) immunocytochemistry and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry. An overall colocalization of NOS immunoreactivity and NADPHd staining was observed. Nitrergic neurons were abundant in the myenteric and outer submucous plexus of the caecum, colon, and rectum. Only a few nitrergic perikarya were seen in the inner submucous plexus of the colon and caecum, whereas a substantially larger number was observed in the rectum. Nitrergic nerve fibers were present in the three ganglionic nerve plexuses. Contrary to the outer longitudinal muscle layer and the mucosal region, the circular muscle layer received a dense nitrergic innervation. The nitrergic nerve cells were variable in size and shape, and several displayed vasoactive intestinal polypeptide (VIP) immunoreactivity (IR). Retrograde tracing studies revealed the existence of nitrergic neurons that project to the caudal (inferior) mesenteric ganglion. They were observed in the myenteric and outer submucous plexus of the transverse and descending colon and the rectum. These observations strongly suggest that several subpopulations of NO-synthesizing neurons, namely, motor neurons and interneurons, should be distinguished in the porcine large intestine, thereby emphasizing the importance of NO as a biologically active mediator.

Neuroepithelial endocrine cells in the lung of the lungfish Protopterus aethiopicus. An electron- and fluorescence-microscopical investigation

The occurrence and distribution of neuroepithelial endocrine (NEE) cells was demonstrated electron- and fluorescence-microscopically in the lungfish Protopterus aethiopicus. They were only found to occur solitarily in the basal part of the cilio-mucous epithelium which is restricted to the pneumatic duct and adjacent parts of the common anterior chamber. The NEE cells show a yellow, formaldehyde-induced fluorescence. Electron-microscopically, all the NEE cells are characterized by membrane-bound electron-dense secretory granules with varying diameters, ranging from 75 to 150 nm. These granules are distributed throughout the cytoplasm with a higher concentration in the basal region. The NEE cells were regularly found to contain paracrystalline inclusions with a tubule-like substructural arrangement. A small part of the NEE cells appeared to reach the luminal surface by means of a long slender process bearing specialized beaded microvilli on its apical pole. Intraepithelial nerve fibres, with the ultrastructural characteristics of afferent fibres, were found running parallel to the airway surface. Nerve profiles, largely resembling the latter, can be seen in the proximity of the basolateral plasma membrane of the NEE cells. In addition, nerve terminals containing an aggregation of small clear vesicles are in close contact with the NEE cells. In conclusion, it appears that, as has so far been assumed in higher vertebrates, the NEE cells in the lung of Protopterus may perceive changes in the airway gases whereupon they could respond by releasing a chemical modulator, influencing contacting afferent nerve terminals or nearby smooth muscle bundles. Furthermore, intraepithelial nerve fibres or NEE cells might be stretch-sensitive.

Fluorescence microscopic study of the architecture and structure of an adrenergic network in the plexus myentericus (Auerbach), plexus submucosus externus (Schabadasch) and plexus submucosus internus (Meissner) of the porcine small intestine

The distribution of adrenergic fibres in the ganglionated plexuses of the porcine small intestine has been made on air-dried stretch preparations using the glyoxylic acid fluorescence method. Adrenergic fluorescent fibres occur in the ganglia and internodal strands of the three fundamental ganglionated plexuses: the myenteric plexus (Auerbach) and the two superimposed meshworks of the plexus submucosus , i.e. the plexus submucosus externus ( Schabadasch ) and the plexus submucosus internus (Meissner). The plexus Auerbach consists of densely glyoxylic acid induced fluorescent (GIF) elongated ganglia with in general a longitudinal axis running parallel to the circular muscle layer and large dense interconnecting fibre tracts with primary, secondary and tertiary subdivisions. In the ganglia, the fibres are varicose, forming large fluorescent 'baskets' which might be related to the occurrence of well defined enteric neurones. The plexus Schabadasch can be distinguished from the plexus Meissner by its size, strongly fluorescent ganglia and broad densely fluorescent internodal strands. The pattern of fluorescing ring-like formations at the margin and out of the nodes, clearly present in the Auerbach and Schabadasch plexuses, completely lack in the plexus Meissner, the latter being narrow-meshed with smaller fluorescent 'baskets', indicating that the corresponding neurones are smaller in size. In the ganglionic nodes of all three plexuses the axons display comparatively more varicosities than in the fibre tracts. Each of the three main ganglionated enteric plexuses are quite different with regard to the pattern of the adrenergic network both in the ganglia and in the strands.

Excitatory synaptic inputs on myenteric Dogiel type II neurones of the pig ileum

The synaptic input on myenteric Dogiel type II neurones (n = 63) obtained from the ileum of 17 pigs was studied by intracellular recording. In 77% of the neurones, electrical stimulation of a fibre tract evoked fast excitatory postsynaptic potentials (fEPSPs) with an amplitude of 6 +/- 5 mV (mean +/- S.D.) and lasting 49 +/- 29 ms. The nicotinic nature of the fEPSPs was demonstrated by superfusing hexamethonium (20 microM). High-frequency stimulation (up to 20 Hz, 3 seconds) did not result in a rundown of the fEPSPs, and did not evoke slow excitatory or inhibitory postsynaptic potentials. The effects of neurotransmitters, possibly involved in these excitatory responses, were investigated. Pressure microejection of acetylcholine (10 mM in pipette) resulted in a fast nicotinic depolarisation in 67%(18/27) of the neurones (13 +/- 9 mV, duration 7.0 +/- 7.2 seconds) as did 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) application (10 mM; 14 +/- 10 mV, duration 4.1 +/- 2.8 seconds) in 76% of the cells. The fast nicotinic response to acetylcholine was sometimes (6/27) followed by a slow muscarinic depolarisation (8 +/- 4 mV; duration 38.7 +/- 10.8 seconds). Immunostaining revealed 5-hydroxytryptamine hydrochloride (5-HT)- and calcitonin gene-related peptide (CGRP)-positive neuronal baskets distributed around and in close vicinity to Dogiel type II neuronal cell bodies. Microejection of 5-HT (10 mM) resulted in a fast nicotinic-like depolarisation (12 +/- 6 mV, duration 3.0 +/- 1.3 seconds) in 4 of 8 neurones tested, whereas microejection of CGRP (20 mM) gave rise to a slow muscarinic-like depolarisation (6 +/- 2 mV, duration 56.0 +/- 27.5 seconds) in 8 of 12 neurones tested. In conclusion, myenteric Dogiel type II neurones in the porcine ileum receive diverse synaptic input. Mainly with regard to the prominent presence of nicotinic responses, these neurones behave contrary to their guinea pig counterparts.

Neuromedin U-immunoreactivity in the nervous system of the small intestine of the pig and its coexistence with substance P and CGRP

In the small intestine of the pig, neuromedin U (NMU)-immunoreactivity was mainly confined to the nerve plexus of the inner submucosal and mucosal regions. After colchicine treatment, a high number of immunoreactive nerve cell bodies was observed in the plexus submucosus internus (Meissner), whereas only a low number was found in the plexus submucosus externus (Schabadasch). The plexus myentericus as well as the aganglionic nerve meshworks in the circular and longitudinal smooth muscle layers almost completely lacked NMU-immunoreactivity. Double-labeling experiments demonstrated the occurrence of distinct NMU-containing neuron populations in the plexus submucosus internus: (1) relatively large type-II neurons revealing immunoreactivity for NMU and calcitonin gene-related peptide (CGRP) and/or substance P (SP); (2) a group of small NMU- and SP-immunoreactive neurons; (3) a relatively low number of small neurons displaying immunoreactivity for NMU but not for SP. Based on its distributional pattern, it is concluded that NMU plays an important role in the regulation and control of mucosal functions.

Immunoreactivity for calcitonin gene-related peptide in neuroepithelial bodies of the newborn cat

Immunoreactivity for calcitonin gene-related peptide is demonstrated for the first time in neuroepithelial bodies in the lung of newborn cats after Bouin fixation and embedding in paraffin. The intense staining clearly identifies these bodies at the level of bronchioli and alveoli. Occasionally, single neuroepithelial endocrine cells, displaying immunoreactivity for calcitonin gene-related peptide are observed. In the kitten lung, identification and localization of neuroepithelial bodies after immunocytochemical staining for calcitonin gene-related peptide are superior to the analysis based on other techniques, i.e., the argyrophilic reaction, periodic acid Schiff-lead hematoxylin method, and immunocytochemical staining for serotonin. The serial-section technique revealed that in neuroepithelial bodies of the newborn kitten lung, immunoreactivity for calcitonin gene-related peptide coexists with immunoreactivity for serotonin in individual cells. The functional significance of the calcitonin gene-related peptide in neuroepithelial bodies remains to be elucidated.