A comparative immunocytochemical study of the gastro-entero-pancreatic (GEP) endocrine system in a stomachless and a stomach-containing teleost

Tenets in Microbial Endocrinology: A New Vista in Teleost Reproduction

Climate vulnerability and induced changes in physico-chemical properties of aquatic environment can bring impairment in metabolism, physiology and reproduction in teleost. Variation in environmental stimuli mainly acts on reproduction by interfering with steroidogenesis, gametogenesis and embryogenesis. The control on reproductive function in captivity is essential for the sustainability of aquaculture production. There are more than 3,000 teleost species across the globe having commercial importance; however, adequate quality and quantity of seed production have been the biggest bottleneck. Probiotics are widely used in aquaculture as a growth promoter, stress tolerance, pathogen inhibition, nutrient digestibility and metabolism, reproductive performance and gamete quality. As the gut microbiota exerts various effects on the intestinal milieu which influences distant organs and pathways, therefore it is considered to be a full-fledged endocrine organ. Researches on Gut-Brain-Gonad axis (GBG axis) and its importance on physiology and reproduction have already been highlighted for higher mammals; however, the study on fish physiology and reproduction is limited. While looking into the paucity of information, we have attempted to review the present status of microbiome and its interaction between the brain and gut. This review will address a process of the microbiome physiological mechanism involved in fish reproduction. The gut microbiota influences the BPG axis through a wide variety of compounds, including neuropeptides, neurotransmitter homologs and transmitters. Currently, research is being conducted to determine the precise process by which gut microbial composition influences brain function in fish. The gut-brain bidirectional interaction can influence brain biochemistry such as GABA, serotonin and tryptophan metabolites which play significant roles in CNS regulation. This review summarizes the fact, how microbes from gut, skin and other parts of the body influence fish reproduction through the Gut-Brain-Gonad axis.

Antigen sampling in the fish intestine

Antigen uptake in the gastrointestinal tract may induce tolerance, lead to an immune response and also to infection. In mammals, most pathogens gain access to the host though the gastrointestinal tract, and in fish as well, this route seems to be of significant importance. The epithelial surface faces a considerable challenge, functioning both as a barrier towards the external milieu but simultaneously being the site of absorption of nutrients and fluids. The mechanisms allowing antigen uptake over the epithelial barrier play a central role for maintaining the intestinal homeostasis and regulate appropriate immune responses. Such uptake has been widely studied in mammals, but also in fish, a number of experiments have been reported, seeking to reveal cells and mechanisms involved in antigen sampling. In this paper, we review these studies in addition to addressing our current knowledge of the intestinal barrier in fish and its anatomical construction.

Cholecystokinin: how many functions? Observations in seabreams

A short overview on the regional distribution of the gastro-intestinal peptide hormone cholecystokin (CCK) in fish is presented. In particular, the results of molecular and immunological studies on seabreams, Diplodus puntazzo and Diplodus sargus, are reported, which, by demonstrating CCK in the hindgut, open new questions regarding the functional role of this hormone in that part of the intestine. The putative involvement of hindgut CCK in the feedback control of digestive processes was tested by measuring CCK gene and protein expression in fed and fasted fish. The results of this study led to hypothesize different roles for the two CCK isoforms in D. sargus, one of which related to regulation of digestive processes from pyloric caeca through hindgut. On the other hand, a functional role alternative to regulation of digestive processes may be inferred for the other isoform.

Characterization of the endocrine, digestive and morphological adjustments of the intestine in response to food deprivation and torpor in cunner, Tautogolabrus adspersus

The cunner, Tautogolabrus adspersus, is a marine teleost endemic to the cold waters of the Northwest Atlantic Ocean. The cunner is non-migratory and is known for its remarkable ability to endure the freezing winter months with little to no food by entering a torpid/dormant state. To evaluate the physiological strategies employed by the cunner's intestinal tract to withstand food deprivation, fish were sampled for their gut after a four-week period of acute food deprivation during their summer (active/feeding) state, as well as after 4months of overwinter fasting. Digestive capacity was evaluated by measuring digestive enzyme activity and related mRNA transcript expression for trypsin, alkaline phosphatase, alanine aminopeptidase and lipase. In order to assess how gut hormones affect/are affected by acute fasting and torpor, we examined the intestinal mRNA expression of several putative appetite regulators, i.e. CCK, apelin, orexin and mTOR. Short-term summer fasting induced a reduction in the activity, but not the transcript expression, of all digestive enzymes examined as well as a reduction in gut apelin mRNA. Torpor induced a reduction in the activity of all enzymes with the exception of alanine aminopeptidase, and a decrease in mRNA levels of alanine aminopeptidase, orexin, CCK and mTOR. Our results suggest that both acute fasting and long-term fasting induce a reduction in the intestinal function of cunner, as evidenced by an overall decrease in the activities of digestive enzymes and mRNA expression of several factors involved in feeding and digestion.

Cholecystokinin in white sea bream: molecular cloning, regional expression, and immunohistochemical localization in the gut after feeding and fasting

BACKGROUND

The peptide hormone cholecystokinin (CCK), secreted by the midgut, plays a key role in digestive physiology of vertebrates including teleosts, by stimulating pancreatic secretion, gut motility, and gallbladder contraction, as well as by delaying gastric emptying. Moreover, CCK is involved in the regulation of food intake and satiation. Secretion of CCK by the hindgut is controversial, and its biological activity remains to be elucidated. The present paper addresses the regional distribution of intestinal CCK in the white sea bream, Diplodus sargus, as well as the possible involvement of hindgut CCK in digestive processes.

METHODOLOGY/PRINCIPAL FINDINGS

Full-lengths mRNAs encoding two CCK isoforms (CCK-1 and CCK-2) were sequenced and phylogenetically analyzed. CCK gene and protein expression levels in the different gut segments were measured 3 h and 72 h after feeding, by quantitative real-time RT-PCR and Western blot, respectively. Moreover, endocrine CCK cells were immunoistochemically detected. Fasting induced a significant decrease in CCK-2 in all intestinal segments, including the hindgut. On the other hand, no significant difference was induced by fasting on hindgut CCK-1.

CONCLUSIONS/SIGNIFICANCE

The results demonstrated two CCK isoforms in the hindgut of D.sargus, one of which (CCK-2) may be involved in the feedback control of uncompleted digestive processes. On the other hand, a functional role alternative to regulation of digestive processes may be inferred for D.sargus CCK-1, since its expression was unaffected by feeding or fasting.

Autonomic control of gut motility: a comparative view

Gut motility is regulated to optimize food transport and processing. The autonomic innervation of the gut generally includes extrinsic cranial and spinal autonomic nerves. It also comprises the nerves contained entirely within the gut wall, i.e. the enteric nervous system. The extrinsic and enteric nervous control follows a similar pattern throughout the vertebrate groups. However, differences are common and may occur between groups and families as well as between closely related species. In this review, we give an overview of the distribution and effects of common neurotransmitters in the vertebrate gut. While the focus is on birds, reptiles, amphibians and fish, mammalian data are included to form the background for comparisons. While some transmitters, like acetylcholine and nitric oxide, show similar distribution patterns and effects in most species investigated, the role of others is more varying. The significance for these differences is not yet fully understood, emphasizing the need for continued comparative studies of autonomic control.

Insulin expression in the brain and pituitary cells of tilapia (Oreochromis niloticus)

While the presence of immunoreactive insulin in the central nervous system of many vertebrate species is well known, the origin of brain insulin is still debated. In this study, we applied RT-PCR, quantitative RT-PCR (qRT-PCR), and Northern hybridization to examine expression of the insulin gene in different tissues of an adult teleost fish, the Nile Tilapia (Oreochromis niloticus). We found that the insulin gene is transcribed at a high level in Brockmann bodies (pancreatic islet organs) and at a low level in the brain and pituitary gland. In the brain, insulin transcripts were detected in all areas by qRT-PCR and in situ hybridization. The highest level of insulin mRNA was found in the hypothalamus. The level of insulin transcription in the pituitary gland was 6-fold higher than that in the brain and 4.6-fold higher than that in the hypothalamus. Furthermore, insulin mRNA and immunoreactive insulin-like protein was detected in the pituitary gland using in situ hybridization, immunohistochemistry, and Western blot analysis. Our results indicate that in adult tilapia insulin expression is not restricted to the endocrine pancreatic cells, but also occurs in endocrine cells of the pituitary gland and in the neuronal cells of the brain, suggesting that the brain/pituitary gland might represent extrapancreatic origin of insulin production.

Immunohistochemical study on distribution of endocrine cells in gastrointestinal tract of flower fish (Pseudophoxinus antalyae)

AIM: To detect distribution and relative frequency of endocrine cells in gastrointestinal tract of flower fish (Pseudophoxinus antalyae).

METHODS: The intestinal tract of flower fish was divided into four portions from proximal to distal; the enlarged area after oesophagus and anterior, middle and posterior intestine. Immunohistochemical method using the peroxidase anti-peroxidase complex was employed. All antisera between four portions of flower fish were compared using ANOVA.

RESULTS: Eleven types of gut endocrine cells were determined; they were immunoreactive for calcitonin gene related peptide, substance P, vasoactive intestinal peptide, bombesin, somatostatin-14, secretin, TrkA, TrkB, TrkC, neurotensin, neuropeptide Y, which were found in almost all portions of the gastrointestinal tract.

CONCLUSION: The regional distribution and relative frequency of immunoreactive cells in the flower fish, Pseudophoxinus antalyae, are essentially similar to those of other fish.

Ontogeny of the endocrine pancreatic cells of the gilthead sea bream, Sparus aurata (Teleost)

The development of the gilthead sea bream, Sparus aurata, endocrine pancreas was studied from hatching to 114 days, using immunocytochemical techniques. Bonito insulin (INS)-, synthetic somatostatin-14 (SS-14)-, salmon somatostatin-25 (SS-25)-, mammalian somatostatin-28 (1-12) (SS-28)-, porcine glucagon (GLU)-, glucagon-like peptide-1 (1-19) (GLP-1)-, synthetic porcine peptide tyrosine tyrosine (PYY)-, and neuropeptide Y (NPY)-like immunoreactivities were demonstrated. The different types of endocrine cells appear at distinct stages of development and differ in their arrangement. The coexistence of INS and SS-25 immunoreactivities was demonstrated in the cells of one strand or primordial cord and a primordial islet that appeared close to the dorsal epithelium of the anterior region of the undifferentiated gut or next to the gut at hatching and one day after hatching, respectively. INS- and SS-25- immunoreactive (ir) cells were located in the core and at the periphery of the single islet found in 2-day-old larvae, while SS-28-ir cells were found in the single islet in 4- to 11-day-old larvae. GLU/GLP-1-ir cells were located next to the outer SS-25-ir cells in the single islet of 12- and 16-day-old larvae. SS-14/SS-25- and SS-14/SS-28-ir cells were detected in the outer region and in the inner area of the single islet, respectively, in 17- to 23-day-old larvae. One big islet and several small islets and isolated or clustered cells next to the pancreatic duct were present in 24- and 25-day-old larvae. The islets were similar in cell composition to the single islet seen in the previous stage, while the isolated and grouped cells showed the coexistence of INS and diverse SSs immunoreactivities. Nerve fibers showing PYY immunoreactivity were identified in the islets from 17 days onwards. In 30- to 44-day-old larvae, GLU and NPY immunoreactivities coexisted in a few cells at the periphery of some small islets. PYY-ir cells were first detected at day 51. One big islet, several intermediate islets and numerous small islets were present from 51-day-old-larvae to juveniles. GLU was colocalized with PYY and NPY in a few cells in a small peripheral area in the big islet and a few intermediate islets. The outer region of small islets and other intermediate islets showed the complete coexistence of GLU, PYY, and NPY.

Hormonal regulation of the fish gastrointestinal tract

The gastrointestinal tracts (GIT) of fish and other vertebrates are challenged with a diversity of functional demands caused by changes and differences in dietary inputs and environmental conditions. This contribution reviews how hormonal regulation plays an essential role in modulating the GIT functions of fish to match changes in functional demands. Exemplary is how hormones produced by the GIT, the associated organs (e.g., pancreas), and other sources (e.g., hypothalamus, adrenal cortex, thyroid, gonads) modulate the digestive processes (motility, secretion, and nutrient absorption) in response to dietary inputs. Hormones regulate the other GIT functions of osmoregulation (secretion and absorption of electrolytes and water), immunity, endocrine secretions, metabolism, and the elimination of toxic metabolites and environmental contaminants to match changes in environmental conditions and physiological states. Although the regulatory molecules and associated signaling pathways have been conserved during evolution of the vertebrate GIT, the specific responses often vary among fish with different feeding habits and from different environments, and can differ from those described for mammals.

Immunohistochemical study on the endocrine cells in gut of the stomachless teleost, Zacco platypus (Cyprinidae)

The regional distribution and relative frequency of neurohormonal peptides-producing cells were demonstrated in the gut of the stomachless teleost, Zacco platypus (Temminck et Schegel), using 10 types of specific antisera raised against mammalian regulatory peptides. The gut of Z. platypus was divided into five portions from proximal to distal (segment I-V). Most of immunoreactive cells in the epithelial lining portion, between epithelial cells, were generally spherical or spindle shape having long cytoplasmic process that was reached via the lumen while cells round in shape were found in the basal portions of epithelial lining occasionally. Serotonin-, somatostatin-, glucagon-, cholecystokinin (CCK)-8 and pancreatic polypeptide (PP)-immunoreactive cells were observed in this study. However, no chromogranin A-, secretin-, vasoactive intestinal peptide (VIP)-, substance P- and bombesin-immunoreactive cells were found. Serotonin-immunoreactive cells were demonstrated throughout the entire gut tract and occurred more frequently than other cells. Somatostatin-immunoreactive cells were restricted to proximal segments of the gut (segment I-III) with rare frequencies, and glucagon-immunoreactive cells were demonstrated in the proximal segments of the gut (segment I, II) with moderate to few occurrences. CCK-8-immunoreactive cells were found throughout the whole intestinal tract except for most proximal segment (segment I) with moderate to few frequencies and PP-immunoreactive cells were demonstrated in the proximal to middle segments, segment I-III, with a few, rare and rare frequencies, respectively.

An immunohistochemical study on the neuroendocrine system in the alimentary canal of the brown trout, Salmo trutta, L., 1758

Several neurohormonal peptides of the gastrointestinal system of fish have been revealed by immunohistochemical methods. Among salmonids, the rainbow trout, Oncorhynchus mykiss (Walbaum) is the most studied species, whereas the informations about other species of the taxonomic group are lacking. The regional distribution and relative densities of cells belonging to the neuroendocrine system have been in this paper demonstrated in the gut of the brown trout, Salmo trutta Linnaeus. In the gastric mucosa, endocrine cells were detected, which were immunoreactive to bombesin-, gastrin-, and secretin-antisera. Endocrine cells containing gastrin-, bombesin-, cholecystokinin-8-, glucagon-, and leptin-like immunoreactivities were present in the pyloric caeca and intestine. The pancreatic endocrine islets contained glucagon-, and, possibly, secretin-like-immunoreactive endocrine cells, as well as a contingent of galanin-like-immunoreactive nerve fibres. The exocrine pancreatic parenchyma showed bombesin-like-immunoreactive nerve fibres. Within the tested regulatory peptides, bombesin and leptin were observed in both endocrine cells and nerve cell bodies and fibres. Leptin was in addition detected in epithelial cells of the gastric glands. In the brown trout we have never observed any immunoreactivity to the VIP antiserum (either in the stomach or in the intestine). Some special structural patterns (in particular those ones related to galanin- and leptin-immunohistochemical data) have thus been detected for the first time in the brown trout, and provide further data for a better knowledge of gut morpho-functional aspects in this economically important fish.

Distribution of cholecystokinin-immunoreactive cells in the digestive tract of the larval teleost, ayu, Plecoglossus altivelis

The ontogenetic development of cholecystokinin-immunoreactive (CCK-IR) cells was studied in larval ayu, Plecoglossus altivelis. This species has a straight digestive tract during the larval phase. CCK-IR cells were present in all the larvae from the day of hatching (0 days after hatching, DAH). An immunoreaction to anti-trypsinogen antibody was also detected in the pancreas at this stage. The distribution pattern of the CCK-IR cells was quantified by recording the location of CCK-IR cells at 1, 16, and 76 DAH. Although the number of CCK-IR cells increased during development, the distribution pattern of CCK-IR cells did not change until 76 DAH. The CCK-IR cells were scattered throughout the midgut, with the exception of the regions adjacent to the pyloric and rectal sphincters. No CCK-IR cells were detected in the foregut or the hindgut. This distribution pattern differs from species with rotated digestive tracts, whose CCK-IR cells are only found in the anterior part of the midgut. CCK-IR cells seem to be located in regions where the ingested food is retained and thus can easily receive chemical signals from the food and the digestive process in order to control the release of the hormone.

Influence of enteric helminths on the distribution of intestinal endocrine cells belonging to the diffuse endocrine system in brown trout, Salmo trutta L

The presence of intestinal helminths in the alimentary canal of brown trout, Salmo trutta L., can alter the number of cells that synthesize modulatory peptides. A total of 167 brown trout were collected from tributaries of the River Brenta (northern Italy), of which 119 (71.3%) specimens were infected with enteric helminths, 28 with the acanthocephalan Pomphorhynchus laevis Müller, 1776 with intensity of infection ranging from 1 to 162 (18.57 +/- 30.79) worms per host and 67 fish with the cestode Cyathocephalus truncatus Pallas, 1781. Intensity of infection with C. truncatus ranged from 1 to 85 (6.87 +/- 12.59) per fish. In 24 fish there were concurrent infections of both species of helminths. The caecal and middle regions of the intestine were the most heavily parasitized. Immunohistochemical tests showed a decrease in endocrine cells (ECs) of the diffuse endocrine system (DES) positive to gastrin, cholecystokinin-8, bombesin and secretin antisera in the intestine of the infected trout. The number of ECs immunoreactive to anti-glucagon serum did not show differences in the digestive tract of uninfected brown trout and in conspecifics parasitized with P. laevis. The density of cells containing glucagon-like material was low in the fish parasitized with C. truncatus. The results suggest that endoparasitic helminths induce alterations in the DES of infected S. trutta.

Glucagon-like peptide isolated from the eel intestine: effects on atrial beating

A new glucagon-like peptide was isolated from the intestine of the eel Anguilla japonica. The primary structure was determined by sequence analysis after cleavage with lysyl endopeptidase, quantitative amino acid analysis and fast atom bombardment mass spectrometry as HSQGTFTNDY10SKYLETRRAQ20DFVQWLMNSK30RSGGPT. Since its structure is similar to that of oxyntomodulins (OXMs) reported in various vertebrates, we named this peptide eel oxyntomodulin (eOXM). We found that eOXM enhanced the contractile force and the beating rate of the eel atrium in a dose-dependent manner. These effects of eOXM were not inhibited by betaxolol, a β1-adrenoceptor antagonist, indicating that the actions of eOXM were independent of those of adrenaline. eOXM enhanced the intracellular Ca2+ concentration of the myocardium. The contractility of the eel atrium was greatly reduced after omitting Ca2+ from the bathing medium or after treatment with verapamil, a Ca2+ channel blocker. After inhibiting Ca2+ entry under these conditions, the inotropic effect of eOXM was markedly reduced, but the chronotropic effect was not altered significantly. These results indicate that the inotropic effect of eOXM is via a stimulation of Ca2+ influx but that the chronotropic effect may be independent of extracellular Ca2+.

Immunocytochemical identification and localization of APUD cells in the gut of seven stomachless teleost fishes

AIM: To study the cell types, localization, distribution density and morphology of APUD cells in the intestinal mucosa of stomachless teleost fishes.

METHOD: By using the peroxidase-antiperoxidase complex ( PAP ) immunocytochemical staining technique the identification, localization and morphology of immunoreactive (IR) endocrine cells seattered in the intestinal mucosa of grass carp (Cyenopharyngodon idellus), black carp (Mylopharyngodon piceu s) and common carp (Cyprinus carpio) were investigated with 20 kinds of an tisera prepared against mammalian peptide hormones of APUD cells, and likewise by using avidin-biotin-peroxidase complex (ABC) method those of silver carp (Hypophthalmichthys molitrix), bighead (Aristichthys nobilis), silver crucian carp (Carassius gibelio) and bluntnose black bream (Megalobrama amblyoce phala) were also studied with 5 different antisera. The replacement of the first antiserum by phosphate buffered saline (PBS) was employed as a control. IR endocrine cells were counted with a square-mesh ocular micrometer from 10 fields selected randomly in every section of each part of the intestine specimen. The average number of IR endocrine cells per mm² was counted to quantify their dis tribution density.

RESULT: Gastrin (GAS), Gastric inhibitory peptide (GIP), glucagon (GLU), glucagon-like immunoreactants (GLI), bovine pancreatic polype ptide (BPP), leucine-enkephalin (ENK) and substance P (SP)-IR endocrine ce lls were found in the gut of grass carp, black carp and common carp, and somatos tatin (SOM)-IR endocrine cells were only seen in common carp. GAS, GIP and GLU-IR endocrine cells were found in the intestinal mucosa of silver carp, bigh ead, silver crucian carp and bluntnose black bream. Most of IR endocrine cells had the higher distribution density in the foregut and midgut, and were longer in shape. They had a long apical cytoplasmic process extended to the gut lumen and a basal process extended to adjacent cells or basement membrane and touched with it. Sometimes, the basal cytoplasmic process formed an enlarged synapse-like structure in the contiguous part with basement membrane. This phenomenon provide d new morphological evidence for neuroendocrine and paracrine secretory function of these enteroendocrine cells.

CONCLUTION: At least 8 kinds of IR endocrine cells were found in the gut of stomachless teleost species for the first time in China. These IR e ndocrine cells scattering in the gut mucosa belong to the APUD system. Among the m, the hormones secreted by SP-, ENK-, SOM- and GLU-IR endocrine cells belon g to the peptides of dual distribution in the brain and gut. This provided new evidence for the concept of brain-gut peptide. According to the cell types, dist ribution density, morphological characteristics and variety in shape of APUD cells in the gut of stomachless teleost fishes, it is deemed that the digestive tract of fishes is also an endocrine organ of great importance and complexity.

Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fish

The morphology of the gastroenteropancreatic (GEP) system of fish was reviewed with the objective of providing the phylogenetic and ontogenetic development of the system in this vertebrate group, which includes agnathans and gnathostome cartilaginous, actinoptyerygian, and sarcopterygian fish. Particular emphasis is placed on the fish homolog of the endocrine pancreas of other vertebrates, which is referred to as the islet organ. The one-hormone islet organ (B cells) of larval lampreys is the most basic pattern seen among a free-living vertebrate, with the two-hormone islet organ (B and D cells) of hagfish and the three-hormone islet organ (B, D, and F cells) of adult lampreys implying a phylogenetic trend toward the classic four-hormone islet tissue (B, D, F, and A cells) in most other fish. An earlier stage in the development of this phylogenetic sequence in vertebrates may have been the restriction of islet-type hormones to the alimentary canal, like that seen in protochordates. The relationship of the islet organ to exocrine pancreatic tissue, or its equivalent, is variable among bony, cartilaginous, and agnathan fishes and is likely a manifestation of the early divergence of these piscine groups. Variations in pancreatic morphology between individuals of subgroups within both the lamprey and chondrichthyan taxa are consistent with their evolutionary distance. A comparison of the distribution and degree of concentration of the components of the islet organ among teleosts indicates a diffuse distribution of relatively small islets in the generalized euteleosts and the tendency for the concentration into Brockmann bodies of large (principal) islets (with or without secondary islets) in the more derived forms. The holostean actinopterygians (Amiiformes and Semiontiformes) share with the basal teleosts (osteoglossomorphs, elopomorphs) the diffuse arrangement of the components of the islet organ that is seen in generalized euteleosts. Since principal islets are also present in adult lampreys the question arises whether principal islets are a derived or a generalized feature among teleosts. There is a paucity of studies on the ontogeny of the GEP system in fish but it has been noted that the timing of the appearance of the islet cell types parallels the time that they appear during phylogeny; the theory of recapitulation has been revisited. It is stressed that the lamprey life cycle provides a good opportunity for studying the development of the GEP system. There are now several markers of cell differentiation in the mammalian endocrine pancreas which would be useful for investigating the development of the islet organ and cells of the remaining GEP system in fish.

Co-localization of Trk neurotrophin receptors and regulatory peptides in the endocrine cells of the teleostean stomach

Recently it has been observed that a subpopulation of gut endocrine cells in vertebrates express Trk-like proteins, suggesting that neurotrophins could regulate the synthesis and storage of amines and peptides of these cells. Nevertheless, the peptides and amines present in the endocrine cells that express Trks have not been characterized. In this study we used immunohistochemistry to investigate the occurrence of Trk-like proteins (TrkA-like, TrkB-like and TrkC-like) and the possible co-localization of these with peptides and/or biogenic amines in the endocrine cells of the stomach of three teleost (bass, gilt-head and scorpionfish). No TrkA-like immunoreactivity (IR) was detected in the stomach of these species, whereas TrkB-like IR and TrkC-like IR were observed in numerous cells of the gastric epithelium. TrkB-like immunoreactive cells were present in all three species examined, and were particularly abundant in the blind sac. Conversely, TrkC-like immunoreactive cells were found only in the bass stomach, apparently co-localized with TrkB-like IR. TrkB-like IR was found co-localized with somatostatin IR in scorpionfish, and with somatostatin and CGRP IR in gilt-head and bass. Gastric endocrine cells expressing 5-HT, glucagon, insulin, met-, leu-enkephalin, substance P, PYY, VIP, CCK, NPY, bombesin and motilin were unreactive for Trk-like proteins. The present results provide direct evidence for the occurrence of Trk-like neurotrophin receptor proteins in a subpopulation of the teleostean gastric endocrine cells and suggest that neurotrophins could regulate, as in neurons, the expression of some neuropeptides such as somatostatin and CGRP.

Immunocytochemical characterization of the pancreatic islet cells of the Nile Tilapia (Oreochromis niloticus)

The cellular composition and topography of the pancreatic islet of Oreochromis niloticus, now known to be a donor source for islet xenotransplantation studies, were characterized. Whole tilapia islets were harvested using an enzymatic method and then further digested into single-cell preparations. Cell cytospin preparations of islet cells and paraffin sections of whole islets were stained using antisera against tilapia insulin, human glucagon, salmon somatostatin-25 (SST-25), human somatostatin-14 (SST-14), and salmon peptide tyrosine-tyrosine (PYY) using the immunoperoxidase method. Cell counts, performed on cytospin preparations using a Quantimet 570 computerized image analysis system, revealed that O. niloticus islets contained 78% endocrine cells and 22% immunonegative cells (i. e., mainly nucleated erythrocytes and rare tissue eosinophils). The proportions of immunopositive endocrine cell types were: 42.3% insulin immunopositive cells, 11.5% glucagon immunopositive cells, 23.1% SST-25 immunopositive cells, 21.8% SST-14 immunopositive cells, and 1.3% PYY immunopositive cells. Islet cell topography was evaluated using histologic sections of whole endocrine pancreata including large, medium, and small islets. Round to polygonal insulin immunopositive cells with round central nuclei were distributed in clusters throughout both the principal and the smaller islets. Elongate SST-14 immunopositive cells were closely associated with the clusters of insulin immunopositive cells; both were surrounded by SST-25 immunopositive cells, which were similar in shape to the insulin immunopositive cells. There were elongate glucagon immunopositive cells throughout the islets, whereas the PYY immunopositive cells were restricted to the periphery and to channels of fibrovascular connective tissue penetrating the islets.

Glucagon- and NPY-related peptide-immunoreactive cells in the gut of sea bass (Dicentrarchus labrax L.): a light and electron microscopic study

Glucagon and peptide of the neuropeptide Y (NPY) family immunoreactivities were studied in the gut of sea bass (Dicentrarchus labrax) using antisera against bovine/porcine glucagon, porcine glucagon, glicentin (10-30), bovine pancreatic polypeptide (PP), peptide tyrosine tyrosine (PYY), salmon PYY (sPYY), and NPY. Glucagon-, glicentin-, PYY-, and NPY-immunoreactive (ir) cells were detected in the stomach, and glucagon-, PP-, PYY-, sPYY-, and NPY-ir cells in the intestine. PP, PYY, and NPY immunoreactivities coexisted in intestinal endocrine cells (NPY-like peptide containing cells), in some of which there was also glucagon immunoreactivity. Preabsorption tests indicated that different products of the glucagon gene(s) are probably expressed in the stomach and intestine of sea bass and that the peptides belonging to the NPY family in the endocrine cells of the intestine are more similar to NPY than to other peptides of this family. Glucagon-ir cells in the stomach, and glucagon/NPY-like containing cells in the intestine, were characterized by conventional and immunogold electron-microscopic techniques. The glucagon cells had secretory granules with a clotted content, the gold particles being observed in both the core and the halo. Glucagon/NPY-like cells showed two types of secretory granules differing in size, both of which were immunogold labeled with anti-NPY and anti-sPYY; the smaller granules were weakly immunogold labeled with anti-glucagon.

Ontogeny of serotonin-immunoreactive cells in the gut epithelium of the cloudy dogfish, Scyliorhinus torazame, with reference to coexistence of serotonin and neuropeptide Y

The ontogeny of serotonin (5-HT)-immunoreactive (IR) cells in the gut epithelium of an oviparous elasmobranch, Scyliorhinus torazame, was examined immunohistochemically. 5-HT-IR cells first appeared in the proximal part of the vitellointestinal duct (VID) and in the anterior part of the midgut of the embryo (30 mm in total length). At the 40-mm stage, the cells slightly increased in number and spread to the posterior part of the midgut, but no labeled cells were found in the foregut or hindgut. By the late embryonic (74- and 80-mm) stages, 5-HT-IR cells were markedly increased in number in the spiral intestine and stomach, whereas they were few in the VID and rectum. During these stages, the density of the cells in the inner yolk sac, the derivative of the VID, tended to be increased. This tendency seemed to be consistent in the posthatching juveniles at the 95-mm stage. In juveniles, 125 mm in length and 1.7 months after hatching, the cells further increased in number in the spiral intestine, reaching their adult value. Double immunostaining by the use of anti-5-HT and -neuropeptide Y (NPY) antisera demonstrated that some of the 5-HT-IR cells were also positive for NPY.

Immunohistochemical studies of the endocrine cells within the gastro-entero-pancreatic system of Osteoglossomorpha, an ancient teleostean group

The identification and distribution of endocrine cells within the gastro-entero-pancreatic (GEP) system of five species of the Osteoglossomorpha (Osteoglossum bicirrhosum, Scleropages jardini, Pantodon buchholzi, Notopterus chitala and Gnathonemus petersii) were analyzed by immunohistochemistry. Four immunoreactive cell types were identified within the pancreatic islets (A, B, D, and F cells), using antisera directed against mammalian insulin (m-INS), somatostatins (SST-14, SST-25), and members of the pancreatic polypeptide (aPY, NPY, PYY) and glucagon (GLU, GLP) families. The B cells were located throughout the center of the islets in the five species and, in general, D cells had a similar distribution. However, immunoreactivity to anti-somatostatins varied between four of the species and G. petersii, which showed less intensely stained D cells in the islets, but greater SST immunoreactivity in both the intestinal and the stomach epithelia than in comparable epithelia of other species. For peptides of both the pancreatic polypeptide and the glucagon families, the immunoreactivity was detected at the periphery of the islets, and there was a suggestion of an interfamily colocalization of peptides in some cells. In addition, glucagon family peptides showed a scattered immunoreactivity throughout the central portion of the islets. A moderately abundant number of cells in the intestine were immunoreactive to the PP family antisera in all five species. However, immunoreactivities to GLU, GLP, SST, and m-INS antisera were variable in intestinal cells of the species. Immunoreactivity with sera raised against m-INS and PYY was also observed in the stomach of P. buchholzi. The significance of these findings is discussed in both ontogenetic and phylogenetic contexts with respect to the GEP system in actinopterygian fishes and with respect to the possibility of variable processing of prohormones in the different organs of these osteoglossomorphs.

An immunohistochemical study of the endocrine cells within the pancreas, intestine, and stomach of the gar (Lepisosteus osseus L.)

The distribution and identity of the various endocrine cell types were examined in the pancreas, stomach, and anterior intestine of the phylogenetically ancient actinopterygian, the gar (Lepisosteus osseus L.), using immunohistochemistry. Antisera used were directed against several insulins (INSs) and somatostatins (SSTs), and members of the pancreatic polypeptide (PP, aPY, NPY) and glucagon (GLUC, GLP) families. In the gar pancreas the most pronounced aggregation of islet tissue is among the exocrine acini near the union of extrahepatic common bile duct with the gastrointestinal junction. Four immunoreactive cell types were identified within well-defined islets (A, B, D, and F cells) but immunoreactive cell types were also seen isolated among the exocrine acini. Centrally located B cells were immunoreactive with mammalian and lamprey INS antisera whereas the widely dispersed D cells immunostained with anti-SST-14, -25, and -34. SST was also localized in the epithelium of the pancreatic ducts. There was a colocalization of immunoreactivity for each member of the PP and GLU families at the periphery of each islet to identify F and A cells, respectively. However, colocalization of peptides from both families is suspected for at least some cells. Although the gastric and intestinal mucosae showed a similar pattern of immunoreactivity to GLP and not GLU, they had contrasting immunoreactivity with the two INS antisera. SST immunoreactivity was restricted to the stomach, whereas three of the four PP-family peptides were only immunoreactive in the intestine. Immunoreactivity to the various antisera used in the study imply that there may be an organ-specific processing of preproinsulin, that the gar SST profile may be more similar to agnathan and bowfin rather than either elasmobranch or teleost SSTs, and that only the GLP portion of the preproglucagon gene is expressed in the gastrointestinal mucosa. Our results are consistent with other recent endocrine studies showing that the gar is a widely distinct actinopterygian.

Ontogeny of IGF-1 and the classical islet hormones in the turbot, Scophthalmus maximus

The ontogeny of the endocrine pancreas of a teleost, the turbot (Scophthalmus maximus), was investigated by the use of double immunofluorescence. Clustered insulin (INS)-immunoreactive (IR) cells were observed on the first day after hatching. During the following days, the islet largely increased in size and some smaller islets appeared. All islets consisted only of INS-IR cells. Between day 5 (onset of exogenous feeding) and 7, somatostatin (SOM) and glucagon (GLUC) cells appeared. In the large (principal) islet, the SOM-IR cells intermingled with the INS-IR cells. In the secondary islets, they occurred at the periphery. The GLUC-IR cells were located at the periphery in all islets. Subsequently, two-four additional small principal islets appeared. At day 11, pancreatic polypeptide (PP)-IR cells were present in principal islets and secondary islets. Starting with day 11, in all islets, insulin-like growth factor 1 (IGF-1) immunoreactivity was localized in numerous PP-IR cells and GLUC-IR and some SOM-IR cells. It also occurred in enteroendocrine cells that seemed to contain none of the classical islet hormones. The early appearance of INS correlates with its key role in the regulation of fish protein and lipid metabolism. Islet-derived IGF-1 might inhibit the regulation of INS secretion in a paracrine manner and may be highly involved in growth-promoting processes.

Regulatory peptides in gastric endocrine cells of the rainbow trout Oncorhynchus mykiss: general distribution and colocalizations

The endocrine cells of the rainbow trout (Oncorhynchus mykiss) stomach have been investigated using the immunocytochemical techniques of peroxidase-anti-peroxidase and avidin-biotin-peroxidase complexes on paraffin sections. 33 antisera were tested and eight immunoreactivities were detected: somatostatin-, glucagon- bombesin-, substance P-, serotonin-, met-enkephalin-, CCK/gastrin-, and chromogranin-like containing cells. All of them were present throughout the gastric mucosa except CCK/gastrin-like containing cells that were restricted to the pyloric epithelium. Somatostatin 25 and chromogranin immunoreactive cells are described for the first time in fish stomach. Serotonin immunoreactive cells were also positive for the Grimelius technique and some of them were immunoreactive to anti substance P or anti CCK/gastrin. Immunoreactivities for gastrin 17, gastrin 34 and CCK appeared in the same cells and the absorption controls showed that a molecule containing the carboxi-terminal pentapeptide of this family was present in trout stomach.

Some peptide-like colocalizations in endocrine cells of the pyloric caeca and the intestine of Oncorhynchus mykiss (Teleostei)

The coexistence of immunoreactivities to cholecystokinin, glucagon, glucagon-like peptide 1, salmon pancreatic polypeptide, neuropeptide tyrosine, and peptide tyrosine tyrosine was studied immunocytochemically, revealing for the first time in fish intestine the existence in the same cell of immunoreactivities to cholecystokinin-glucagon/glucagon-like peptide 1, cholecystokinin-salmon pancreatic polypeptide, glucagon/glucagon-like peptide 1-salmon pancreatic polypeptide, glucagon/glucagon-like peptide 1-neuropeptide tyrosine, salmon pancreatic polypeptide tyrosine tyrosine, and glucagon/glucagon-like peptide 1-peptide tyrosine tyrosine. Colocalization of cholecystokinin-salmon pancreatic polypeptide was observed only in the pyloric caeca of the rainbow trout Oncorhynchus mykiss, while the other colocalizations also occurred in proximal and middle intestinal segments. In all cases, endocrine cells immunoreactive to only one of the paired antisera were detected except for anti-glucagon and anti-glucagon-like peptide 1, which always immunostained the same cells.

Endocrine cells and nerves in the pyloric ceca and the intestine of Oncorhynchus mykiss (Teleostei): an immunocytochemical study

The endocrine cells of rainbow trout pyloric ceca and intestine have been investigated immunocytochemically using the avidin-biotin method. Twenty-six antisera were tested and 13 endocrine cell types immunoreacted with antisera to serotonin, somatostatin-25, bombesin, C-flanking bombesin, substance P, salmon PP, NPY, PYY, PP, glucagon, GLP1, Met-enkephalin, and CCK/G. Glucagon and GLP1 immunoreactivities appear in the same cells. Nerves positive to serotonin, substance P, PHI, and VIP were also found. The presence of cells positive to somatostatin-25, C-flanking bombesin, and salmon PP are described for the first time in fish intestine.

Structure and function of a pentapeptide isolated from the gut of the eel

A novel peptide, H-Gly-Phe-Trp-Asn-Lys-OH, was isolated from eel guts. This peptide, termed eel intestinal pentapeptide (EIPP), enhanced the frequency of the spontaneous contractions and increased the basal tone of the circular muscle of the esophagogastric junction. Furthermore, EIPP enhanced the spontaneous contractions of the longitudinal muscle strips of the gut and stomach, and of the circular muscle of the gastro-intestinal junction. The peptide may be a physiological regulatory peptide in the gastro-intestinal tract of the eel.

Pancreatic endocrine cells in sea bass (Dicentrarchus labrax L.) I. Immunocytochemical characterization of glucagon- and PP-related peptides

PP-, PYY-, and glucagon-immunoreactive cells were immunocytochemically identified in the pancreatic islets of Dicentrarchus labrax (sea bass). PYY cells also reacted with anti-PP serum. The specificity control showed that preabsorption of PP antiserum by PYY peptide abolished the immunostaining, while the reaction did not change when the PYY antiserum was preabsorbed by PP. These results suggested the existence of a PP/PYY molecule in the sea bass islets. The islet distribution of PP/PYY-immunoreactive cells differed markedly. Thus, in the principal islet and some intermediate islets few PP/PYY-immunoreactive cells are present (type I islets), whereas in the smaller and some intermediate ones they are numerous (type II islets). Adjacent sections stained by peroxidase-antiperoxidase (PAP) technique and individual sections stained by immunofluorescence double staining showed the coexistence of glucagon and PP/PYY-like immunoreactivities. Both islet types contained cells with PP/PYY coexisting with glucagon peptide, while cells showing solely glucagon immunoreactivity were found in type I islets only.

Distribution of somatostatin and glucagon immunoreactive cells in the gastric mucosa of some cartilaginous fishes

The comparative distribution of somatostatin- and glucagon-like-containing cells in the histomorphologically different gastric mucosae of the cartilaginous fishes Heptranchias perlo, Raja asterias, Scyliorhinus canicula, Squatina aculeata, and Torpedo marmorata was immunocytochemically studied to demonstrate a possible interrelationship between these endocrine cells and/or other endocrine or nonendocrine cells. In the gastric mucosa, these open-type glucagon and somatostatin immunoreactive cells show a double localization with different morphology and interrelationships. At the bottom of gastric pits, which corresponds to a proliferative zone, spindle or pear-shaped immunopositive cells appear rather numerously and are often in close proximity to each other. In gastric glands, triangular or oval immunopositive cells never in contact with each other were detected; their numeric ratio seems to be rather constant even if their numeric frequency and distribution vary according to the histomorphological aspect of selachian gastric glands. Glucagon immunoreactive cells seem to be more related to pepsinogenic cells, while somatostatin immunoreactive cells seem to be more ubiquitous. Both cell types can present basal cytoplasmic processes. From our results we can suggest a possible regulative role exerted by these two peptides on gastric secretion and cell proliferation.

The endocrine cells in the gut of Mugil saliens Risso, 1810 (Teleostei): an immunocytochemical and ultrastructural study

The endocrine cells in the gut of Mugil saliens Risso, 1810 (leaping grey mullet) were investigated by immunocytochemical and electron microscopic techniques. Gastrin-, glucagon-, and somatostatin-immunoreactive cells were identified in the cardiac and cecal stomach regions, located mainly in the lower part of the gastric folds and in the upper part of the glands. Substance P-, somatostatin-, and pancreatic polypeptide (PP)-immunoreactive cells were found between epithelial cells in the pyloric stomach region. Gastrin-, cholecystokinin (CCK)-, gastric inhibitory polypeptide (GIP)-, substance P-, Met-enkephalin- and PP-immunoreactive cells were observed throughout the intestine while only the last three of these appeared in the posterior intestine. Nine types of gastroenteroendocrine cells were ultrastructurally characterized; some of them were related to the cell types immunocytochemically identified.

Immunocytochemical and ultrastructural characterization of coexistence of pancreatic polypeptide and glucagon-like immunoreactivity in the pancreatic endocrine cells of Sparus auratus L. (Teleostei)

Coexistence of pancreatic polypeptide (PP)- and glucagon-like immunoreactivity was demonstrated in the pancreatic endocrine cells of the teleost fish Sparus auratus. An immunofluorescence double-staining method revealed coexistence of glucagon- and PP-like immunoreactivity in endocrine cells of small and intermediate islets. In contrast to small islets, the intermediate ones also contained a variable number of glucagon-immunoreactive cells next to cells having both immunoreactivities. Coexistence of both immunoreactivities could not be observed in endocrine cells of the principal islet, whereas many cells containing glucagon and a few cells containing PP immunoreactivity were found. By an immunogold double-staining method the precise ultrastructural location of each immunoreactivity could be demonstrated. Again, cells containing glucagon- and/or PP-like immunoreactivity were found. Although, only two different types of granules were observed, four distinct cell types could be distinguished. Based on this granule morphology two cell types showing coexistence were found: one cell type, only present in the small islets, showing a different distribution of glucagon and PP immunoreactivity within the granules (predominantly in the center and periphery, respectively) and another cell type with larger granule cores, present in small as well as intermediate islets, having a mixed distribution of both immunoreactivities.

Coexistence of pancreatic polypeptide (PP)-and glucagon-immunoreactivity in pancreatic endocrine cells of mouse

Immunocytochemical double staining techniques were used to study PP- and glucagon-like-immunoreactivity in pancreatic endocrine cells of mouse. An antiserum against FMRFamide appeared to react with all PP-immunoreactive endocrine cells. With fluorescence microscopy most PP/FMRFamide-immunoreactive cells also showed glucagon-immunoreactivity, but cells containing only PP- or glucagon-like substances were found as well. The proportion of cells containing PP-, glucagon, and both immunoreactivities varied strongly from islet to islet in all parts of the pancreas. Using an electron microscopical immunogold double staining procedure on Lowicryl-embedded pancreas, PP/FMRFamide- and glucagon-immunoreactivity appeared to be present in the majority of endocrine A cells; both immunoreactivities were randomly distributed within the granules of these cells. Cells containing only PP/FMRFamide- or glucagon-immunoreactivity were also found. Glucagon- and a faint FMRFamide-immunoreactivity was also observed in osmicated epon-embedded tissue. Independent of their immunoreactivity all positive cells showed the same round electron dense secretory granules.