Neuropeptides in the fish gut. An immunohistochemical study of evolutionary patterns

Neurochemical characterization of myenteric neurons in the juvenile gilthead sea bream (Sparus aurata) intestine

We evaluated the chemical coding of the myenteric plexus in the proximal and distal intestine of gilthead sea bream (Sparus aurata), which represents one of the most farmed fish in the Mediterranean area. The presence of nitric oxide (NO), acetylcholine (ACh), serotonin (5-HT), calcitonin-gene-related peptide (CGRP), substance P (SP) and vasoactive intestinal peptide (VIP) containing neurons, was investigated in intestinal whole mount preparations of the longitudinal muscle with attached the myenteric plexus (LMMP) by means of immunohistochemical fluorescence staining. The main excitatory and inhibitory neurochemicals identified in intestinal smooth muscle were ACh, SP, 5HT, and NO, VIP, CGRP. Some neurons displayed morphological features of ascending and descending interneurons and of putative sensory neurons. The expression of these pathways in the two intestinal regions is largely superimposable, although some differences emerged, which may be relevant to the morphological properties of each region. The most important variances are the higher neuronal density and soma size in the proximal intestine, which may depend on the volume of the target tissue. Since in the fish gut the submucosal plexus is less developed, myenteric neurons substantially innervate also the submucosal and epithelial layers, which display a major thickness and surface in the proximal intestine. In addition, myenteric neurons containing ACh and SP, which mainly represent excitatory motor neurons and interneurons innervating the smooth muscle were more numerous in the distal intestine, possibly to sustain motility in the thicker smooth muscle coat. Overall, this study expands our knowledge of the intrinsic innervation that regulates intestinal secretion, absorption and motility in gilthead sea bream and provides useful background information for rational design of functional feeds aimed at improving fish gut health.

Relative distribution of gastrin-, CCK-8-, NPY- and CGRP-immunoreactive cells in the digestive tract of dorado (Salminus brasiliensis)

The endocrine cells (ECs) of the gastrointestinal mucosa form the largest endocrine system in the body, not only in terms of cell numbers but also in terms of the different produced substances. Data describing the association between the relative distributions of the peptide-specific ECs in relation to feeding habits can be useful tools that enable the creation of a general expected pattern of EC distribution. We aimed to investigate the distribution of ECs immunoreactive for the peptides gastrin (GAS), cholecystokinin (CCK-8), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) in different segments of the digestive tract of carnivorous fish dorado (Salminus brasiliensis) by using immunohistochemistry procedures. The distribution of endocrine cells immunoreactive for gastrin (GAS), cholecystokinin (CCK-8), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) in digestive tract of dorado S. brasiliensis was examined by immunohistochemistry. The results describe the association between the distribution of the peptide-specific endocrine cells and feeding habits in different carnivorous fish. The largest number of endocrine cells immunoreactive for GAS, CCK-8, and CGRP were found in the pyloric stomach region and the pyloric caeca. However, NPY-immunoreactive endocrine cells were markedly restricted to the midgut. The distribution pattern of endocrine cells identified in S. brasiliensis is similar to that found in other carnivorous fishes.

Expression of neuropeptides and anoctamin 1 in the embryonic and adult zebrafish intestine, revealing neuronal subpopulations and ICC-like cells

This immunohistochemical study in zebrafish aims to extend the neurochemical characterization of enteric neuronal subpopulations and to validate a marker for identification of interstitial cells of Cajal (ICC). The expression of neuropeptides and anoctamin 1 (Ano1), a selective ICC marker in mammals, was analyzed in both embryonic and adult intestine. Neuropeptides were present from 3 days postfertilization (dpf). At 3 dpf, galanin-positive nerve fibers were found in the proximal intestine, while calcitonin gene-related peptide (CGRP)- and substance P-expressing fibers appeared in the distal intestine. At 5 dpf, immunoreactive fibers were present along the entire intestinal length, indicating a well-developed peptidergic innervation at the onset of feeding. In the adult intestine, vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating peptide (PACAP), galanin, CGRP and substance P were detected in nerve fibers. Colchicine pretreatment enhanced only VIP and PACAP immunoreactivity. VIP and PACAP were coexpressed in enteric neurons. Colocalization stainings revealed three neuronal subpopulations expressing VIP and PACAP: a nitrergic noncholinergic subpopulation, a serotonergic subpopulation and a subpopulation expressing no other markers. Ano1-immunostaining revealed a 3-dimensional network in the adult intestine containing multipolar cells at the myenteric plexus and bipolar cells interspersed between circular smooth muscle cells. Ano1 immunoreactivity first appeared at 3 dpf, indicative of the onset of proliferation of ICC-like cells. It is shown that the Ano1 antiserum is a selective marker of ICC-like cells in the zebrafish intestine. Finally, it is hypothesized that ICC-like cells mediate the spontaneous regular activity of the embryonic intestine.

Neuropeptide localization in nonmammalian vertebrates

Neuropeptides are particularly suited to comparative and evolutionary studies, since they have been highly conserved during evolution. Based on primary amino-acid structure, neuropeptides can be arranged into families and synthesized as multiple molecular variants. They may play different functional roles in different organs or tissues of the same species, but also among species and classes. Immunohistochemistry (IHC) is powerful technique for localizing the molecular expression of proteins in tissues and cells of different classes of vertebrates and has been fully exploited in the study of the mammalian brain. The present chapter provides a detailed description of the protocols routinely used in our laboratory to analyze the presence and distribution of neuropeptides in nonmammalian vertebrate tissues. Single labeling protocols performed by both light and fluorescein IHC, and double labeling protocols using primary antisera raised in different species or in the same species are described. Antibody and method specificity are also discussed in detail.

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.

Enteric co-innervation of esophageal striated muscle fibers: a phylogenetic study

Enteric co-innervation of striated muscle fibers in the esophagus occurs in several mammalian species including humans. However, the functional significance is still unknown. Phylogenetic data may be instrumental in gaining further insight. We examined the bat Glossophaga soricina and the shrew Suncus murinus as representatives for phylogenetically old mammals. As ruminants the antelope Tragelaphus imberbis, the he-goat Capra falconeri and the sheep Ovis aries were selected. As non-mammals the clawed frog Xenopus laevis as representative for the taxon amphibian and the rainbow trout Oncorhynchus mykiss as representative for the taxon fish were included. Histochemistry for nicotinamide adenine dinucleotide phosphate-diaphorase and acetylcholinesterase as well as immunofluorescence for vasoactive intestinal peptide and alpha-bungarotoxin were used to demonstrate enteric nerve fibers and motor endplates, respectively. Motor endplates were associated with enteric nerve fibers in all species investigated, although the rates of co-innervation varied from approximately 10 to 20% in shrew, antelope, he-goat, frog and fish, approximately 40% in bat to nearly 90% in sheep. These results demonstrate that enteric co-innervation, in spite of varying co-innervation rates, is conserved through vertebrate evolution, and underline the significance of this newly discovered innervation component.

Autonomic innervation of the fish gut

The enteric nervous system follows a similar overall arrangement in all vertebrate groups. In fish, the majority of nerve cell bodies are found in the myenteric plexus, innervating muscles, blood vessels and glands. In this review, I describe similarities and differences in size, shape and transmitter content in enteric neurons in different fish species and also in comparison with other vertebrates, foremost mammals. The use of different histological and immunochemical methods is reviewed in a historical perspective including advantages and disadvantages of different methods. Lately, zebrafish have become an important model species for developmental studies of the nervous system, including the enteric nervous system, and this is briefly discussed. Finally, examples of how the enteric nervous system controls gut activity in fish is presented, focussing on the effect on gastrointestinal motility.

Structure and autonomic innervation of the swim bladder in the zebrafish (Danio rerio)

Many teleosts actively regulate buoyancy by using a gas-filled swim bladder, which is thought to be under autonomic control. Here we investigated the swim bladder in the zebrafish to determine possible mechanisms of gas-content regulation. Fluorescently labelled phalloidin revealed myocytes that appeared to form a possible sphincter at the junction of the pneumatic duct and esophagus. Myocytes also formed thick bands along the ventral surface of the anterior chamber and bilaterally along the posterior chamber. Thinner layers of myocytes were located elsewhere. Staining of peroxidase within erythrocytes revealed a putative rete and smaller blood vessels in muscle bands and elsewhere. The antibodies zn-12, a general neuronal marker, and SV2, a synaptic vesicle marker labelling presynaptic terminals, revealed widespread innervation of the swim bladder system. Widespread innervation of the swim bladder was also indicated by acetylcholinesterase histochemistry, but choline acetyltransferase-immunoreactive (-IR) somata and fibers were limited to the junction of the pneumatic duct and esophagus. In contrast, varicose tyrosine hydroxylase-IR fibers innervated muscles and blood vessels throughout the system. Neuropeptide Y-IR somata were located near the junction of the duct and esophagus and varicose fibers innervated muscles and vasculature of the posterior chamber and duct. Vasoactive intestinal polypeptide immunoreactivity was abundant throughout the anterior chamber but sparsely distributed elsewhere. Serotonin-IR fibers and varicosities were located only along blood vessels near the junction of the pneumatic duct and posterior chamber. Our results suggest that the zebrafish swim bladder is a complex and richly innervated organ and that buoyancy-regulating effectors may be controlled by multiple populations of autonomic neurons.

Neuropeptides and the control of food intake in fish

The brain, particularly the hypothalamus, integrates input from factors that stimulate (orexigenic) and inhibit (anorexigenic) food intake. In fish, the identification of appetite regulators has been achieved by the use of both peptide injections followed by measurements of food intake, and by molecular cloning combined with gene expression studies. Neuropeptide Y (NPY) is the most potent orexigenic factor in fish. Other orexigenic peptides, orexin A and B and galanin, have been found to interact with NPY in the control of food intake in an interdependent and coordinated manner. On the other hand cholecystokinin (CCK), cocaine and amphetamine-regulated transcript (CART), and corticotropin-releasing factor (CRF) are potent anorexigenic factors in fish, the latter being involved in stress-related anorexia. CCK and CART have synergistic effects on food intake and modulate the actions of NPY and orexins. Although leptin has not yet been identified in fish, administration of mammalian leptin inhibits food intake in goldfish. Moreover, leptin induces CCK gene expression in the hypothalamus and its actions are mediated at least in part by CCK. Other orexigenic factors have been identified in teleost fish, including the agouti-related protein (AgRP) and ghrelin. Additional anorexigenic factors include bombesin (or gastrin-releasing peptide), alpha-melanocyte-stimulating hormone (alpha-MSH), tachykinins, and urotensin I. In goldfish, nutritional status can modify the expression of mRNAs encoding a number of these peptides, which provides further evidence for their roles as appetite regulators: (1) brain mRNA expression of CCK, CART, tachykinins, galanin, ghrelin, and NPY undergo peri-prandial variations; and (2) fasting increases the brain mRNA expression of NPY, AgRP, and ghrelin as well as serum ghrelin levels, and decreases the brain mRNA expression of tachykinins, CART, and CCK. This review will provide an overview of recent findings in this field.

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.

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.

Ca2+-recruitment in tachykinin-induced contractions of gut smooth muscle from African clawed frog, Xenopus laevis and rainbow trout, Oncorhynchus mykiss

Changes in intracellular Ca(2+) concentration control many essential cellular functions like the contraction of smooth muscle cells. The aim of this study was to investigate if the tachykinin substance P (SP) engages external Ca(2+)-sources, internal Ca(2+)-sources, or both in the contraction of the gastrointestinal smooth muscle of rainbow trout (Oncorhynchus mykiss) and the African clawed frog (Xenopus laevis). Strip preparations made of either longitudinal smooth muscle of proximal intestine or circular smooth muscle of cardiac stomach were mounted in organ baths and the tension was recorded via force transducers. Ca(2+)-free Ringer's solution containing the Ca(2+) chelating agent EGTA (2mM) abolished all spontaneous contractions. Exposure to SP in Ca(2+)-free solution decreased the response. Preparations were also treated with the Ca(2+)-ATPase inhibitor thapsigargin (10 microM) during 30 min. Thapsigargin reduced the effect of SP on intestinal longitudinal smooth muscle in rainbow trout and on stomach circular smooth muscle in the African clawed frog and to a less extent in the intestinal longitudinal smooth muscle. The results show that external Ca(2+) is of great importance, but is not the only source of Ca(2+) recruitment in SP-activation of gastrointestinal smooth muscle in rainbow trout and the African clawed frog.

Molecular cloning and expression of cDNA encoding a brain bombesin/gastrin-releasing peptide-like peptide in goldfish

A complementary DNA (cDNA) of 928 bp encoding a bombesin (BBS)/gastrin-releasing peptide (GRP) precursor was identified from goldfish brain. Goldfish BBS/GRP messenger RNA (mRNA) encodes a 157 amino acid precursor, which contains a signal peptide sequence, the 22 amino acid putative BBS/GRP-like peptide, and a carboxy-terminal extension peptide. Reverse transcription-polymerase chain reaction (PCR) (RT-PCR) demonstrated that the mRNA for this precursor has a widespread distribution in goldfish brain, and is also present in skin, gastrointestinal tract, gonad, and gill. Phylogenetic analysis of BBS/GRP-like peptide precursors in vertebrates shows that goldfish BBS/GRP is more closely related to the known GRP precursors than to BBS precursors.

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.

Calcitonin gene-related peptide (CGRP), but not tachykinins, causes relaxation of small arteries from the rainbow trout gut

Possible vasoactive effects on small diameter arteries from the rainbow trout gut of calcitonin gene-related peptide (CGRP-chicken) and different fish tachykinins; substance P (SP-trout), neurokinin A (NKA-trout), scyliorhinin I and II (SCY I and SCY II-dogfish), were investigated. CGRP relaxed precontracted arteries with a pD2 value of 8.3+/-0.2. Relaxation to CGRP 10(-8) M was reduced by 86.4+/-5.2% by the CGRP-1 receptor antagonist CGRP8-37 (10(-6) M), but unaffected by NG-nitro-L-arginine (10(-4) M), indomethacin (10(-6) M) and by removal of the endothelium, suggesting no involvement of nitric oxide, prostaglandins or endothelium-derived factors. A low number of CGRP immunoreactive fibers were present in the arterial wall. The tachykinins (10(-12)-10(-6) M) occasionally contracted the relaxed vessel. No synergistic action of SP on the CGRP-induced response was found. A dense plexus of tachykinin-containing fibers without coexisting CGRP innervated the arterial wall. Tachykinins or CGRP had no effect on small diameter veins, and no such immunoreactivity was found in these vessels. In conclusion, CGRP- and tachykinin-containing fibers innervate trout gut arteries. CGRP probably is vasodilatory, while the function of the tachykinin fibers is unknown.

Molecular cloning and expression of cDNA encoding brain preprocholecystokinin in goldfish

A cholecystokinin (CCK) precursor cDNA of 782 bp was identified from goldfish brain. The open reading frame (369 bp) encodes the 123 amino acid precursor which contains mono- and di-basic amino acid endoproteolytic cleavage, C-terminal alpha-amidation and tyrosyl sulfation sites. Expression studies revealed the presence of preproCCK mRNA in the gastrointestinal tract, pituitary and a wide range of brain areas from the olfactory bulbs to the posterior brain region. We have also confirmed the presence of CCK mRNA in the posterior ventrolateral hypothalamus by in situ hybridization, supporting a role of CCK in feeding behavior and regulation of pituitary hormone secretion.

Nitric oxide in the fish gut

Nitric oxide synthase-positive nerve cells have been found in most vertebrate classes and also some invertebrates, indicating an early evolutionary origin for the enzyme and its function as a neurotransmitter. The general distribution and inhibitory effect on motility of nitric oxide in the fish gut agrees well with studies from other vertebrates, but details may vary between species, suggesting variations in function. The coexistence with vasoactive intestinal polypeptide (VIP)/pituitary adenylate cyclase-activating polypeptide (PACAP) suggests a co-function in fish as in mammals, but this remains to be confirmed.

Vip-induced relaxation of small arteries of the rainbow trout, Oncorhynchus mykiss, involves prostaglandin synthesis but not nitric oxide

Small arteries (internal diameter 376 +/- 69 microns) from the proximal intestine region of the rainbow trout were mounted in a myograph apparatus where changes in isometric tension could be recorded. VIP (vasoactive intestinal polypeptide) caused a concentration-dependent relaxation (10(-9)-3 x 10(-7) M) of vessels precontracted with the alpha-adrenoceptor agonist phenylephrine (10(-5) M). The nitric oxide synthase inhibitor L-NAME (10(-4) M) did not affect the VIP-relaxation, neither did the lipoxygenase inhibitor esculetin (10(-5) M). However, the cyclooxygenase inhibitor indomethacin (10(-6) M) shifted the concentration-response curve significantly to the right. The VIP-relaxation was still present after mechanical removal of the endothelium. Sodium nitroprusside (10(-9)-10(-6) M) caused a concentration-dependent relaxation of the precontracted vessel, indicating the presence of soluble guanylate cyclase in the vascular smooth muscle cells. VIP-immunoreactivity was found in varicose nerve fibers in these vessels, but nitric oxide synthase-immunoreactivity could not be demonstrated. These results suggest that in rainbow trout, as in mammals, VIP is an endogenous vasodilating neuropeptide. No endothelium-dependent mechanism seems to be involved, neither is production of nitric oxide. Instead the relaxation is mediated, at least in part, via prostaglandin synthesis.

Vasoactive intestinal polypeptide immunoreactive nerves in the gill arch of teleost fish, Carassius auratus L

The localization of vasoactive intestinal polypeptide (VIP)-immunoreactive (ir) nerve cell bodies and fibers has been studied in the gill arches of goldfish (Carassius auratus, L.) using the peroxidase-antiperoxidase (PAP) immunohistochemical method. It was found that VIP-ir nerve cell bodies are localized in connective tissue on the oral side of the gill arch; these cells were present as single cells, in couples or as small clusters. Moreover, a dense network of VIP-ir fibers was observed beneath the lining epithelium of the raker cushion. The possible involvement of this peptide in mucus secretion in the gill arches of teleost is discussed.

Co-release of substance P and neurokinin A from the Atlantic cod stomach

The function of tachykinins in the control of gastric motility in the cod, Gadus morhua, was studied using native cod substance P ([Lys1, Arg3, Ile3]SP) and cod neurokinin A ([Ile3, Asn4]NKA). Both cod SP and NKA produced contractions of the vascularly perfused cod stomach, SP being almost 6 times more potent than NKA (pD2-values 7.05 +/- 0.06 and 6.28 +/- 0.09, respectively). The release of tachykinins from the cod stomach was measured in radioimmunoassay, using specific antibodies for the two cod tachykinins. Stimulation of the stomach motility by electrical stimulation of the vagus nerve or infusion of acetylcholine increased the amounts of SP and NKA released into the vascular perfusate. The results suggest that both tachykinins are involved in the excitatory response of the cod stomach produced by vagal and cholinergic stimulation.

Distribution of PACAP (pituitary adenylate cyclase-activating polypeptide)-like and helospectin-like peptides in the teleost gut

Pituitary adenylate cyclase-activating polypeptide (PACAP) and helospectin are two vasoactive intestinal polypeptide (VIP)-related neuropeptides that have recently been demonstrated in the mammalian gut; the aim of this study was to reveal their occurrence and localisation in the gastrointestinal tract, swimbladder, urinary bladder and the vagal innervation of the gut of teleosts, using immunohistochemical methods on whole-mounts and sections of these tissues from the Atlantic cod, Gadus morhua and the rainbow trout, Oncorhynchus mykiss. Both PACAP-like and helospectin-like peptides were present in the gut wall of the two species. Immunoreactive nerve fibres were found in all layers but were most frequent in the myenteric plexus and along the circular muscle fibres. Immunoreactivity was also demonstrated in nerves innervating the swimbladder wall, the urinary bladder and blood vessels to the gut. Immunoreactive nerve cell bodies were found in the myenteric plexus of the gut and in the muscularis mucosae of the swimbladder. In the vagus nerve, non-immunoreactive nerve cells were surrounded by PACAP-immunoreactive fibres. Double staining revealed the coexistence of PACAP-like and helospectin-like peptides with VIP in all visualized nerve fibres and in some endocrine cells. It is concluded that PACAP-like and helospectin-like peptides coexist with VIP in nerves innervating the gut of two teleost species. The distribution suggests that both PACAP and helospectin, like VIP, are involved in the control of gut motility and secretion.

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.

[The effect of cadmium on the intestine of Carassius auratus]

The paper deals with the cadmium effects on the intestinal mucosa of adult Carassius auratus after 7, 14 and 40 days of exposure. After 7 days the mucosa becomes oedematous; the mucous cells show intensive secretion. The cells showing immunoreactivity for the Met- and Leu-enkephalin miss this feature. After 14 days these modifications diminish and after 40 days disappear completely and the mucosa gains its normal characteristics.

Bombesin acts to suppress feeding behavior and alter serum growth hormone in goldfish

The acute effects of a single injection of bombesin (BBS) on feeding behavior and serum growth hormone (GH) levels in goldfish were examined. When injected intraperitoneally (IP), BBS (0.5-100 ng/g) caused a dose-dependent decrease in food intake within 30 and 45 min of administration; maximal suppression was achieved at 50 ng/g BBS and was accompanied by an elevation in serum GH levels. Associated with IP injection of BBS was a pronounced spitting out behavior in which food pellets were taken into the oral cavity but immediately expelled. When injected into goldfish deprived of food for 72 h, 50 ng/g BBS was still potent in suppressing feeding behavior and increasing serum GH. Additionally, IP injection of BBS (10 or 100 ng/g) into groups of fish caused a significant increase in circulating serum GH levels at 1.5 h postinjection. Finally, when injected into the third brain ventricle (ICV), 60 ng/g BBS also caused a suppression in food intake and a concomitant increase in serum GH. Groups of fish injected ICV with 5 or 50 ng/g BBS also exhibited a graded increase in serum GH levels at 45 min postinjection. Overall, these data are the first to demonstrate in any lower vertebrate that a neuropeptide acts to suppress food intake and cause concomitant alterations in circulating serum GH levels, following either peripheral or central administration.

Effects of substance P and distribution of substance P-like immunoreactivity in nerves supplying the stomach of the cod, Gadus morhua

The innervation of the cod stomach by neurons showing substance P-like immunoreactivity (SPLI), and the effect and mechanism of action of substance P (SP) on the vascularly perfused cod stomach and on isolated muscle strip preparations from the pyloric sphincter have been investigated.Infusion of SP produced a contraction of the stomach wall, which could not be blocked by tetrodotoxin, atropine or methysergide, indicating a direct effect on the stomach smooth muscle. Similarly, the contraction produced by SP on preparations from the pyloric sphincter was unaffected by tetrodotoxin.Nerves showing SPLI were frequent in the myenteric plexus of the whole stomach, and in the submucosa and mucosa of the pyloric part of the stomach. SPLI was also observed in fibres in the intestinal branch of the vagus and occasionally in the splanchnic nerves. Ligation of the nerves showed an accumulation of SPLI above as well as below the ligature, being more prominent proximal to the ligature in the vagus and distal to the ligature in the splanchnic nerve. In the vagus nerve, descending and ascending SPLI-fibres were seen surrounding non-reactive cell bodies. No reduction in intensity of the immunoreaction of the neurons in the stomach wall was observed after ligation or sectioning of the vagosympathetic trunk or the splanchnic nerves, nor were SP-levels measured by radioimmunoassay reduced. After denervation of vagal branches close to the stomach wall an insignificant decrease of immunoreactivity was observed in the myenteric plexus. Capsaicin treatment had no conclusive effect on the distribution of SPLI.It is concluded that the innervation showing SPLI may be of intrinsic as well as extrinsic origin, with pathways in both vagal and splanchnic branches. Only a direct effect of SP on the smooth muscle could be demonstrated.

Immunohistochemistry and nerve lesion experiments on the methionine-enkephalin immunopositive neurons in the small intestine of the bullfrog (Rana catesbeiana)

Nerve elements in the small intestine of the bullfrog. Rana catesbeiana, were studied by immunohistochemistry with anti-methionine enkephalin antisera and by nerve lesion experiments, using laser irradiation. Methionine-enkephalin immunopositive nerve fibers occur in the myenteric plexus, circular muscle layer, submucosa, and mucosa. Immunopositive nerve cell bodies in the myenteric plexus have dendrite-like and a long axon-like processes. In the froglet (3 months after metamorphosis), these axon-like processes lead posteriorly in the nerve strand of the myenteric plexus. Some bifurcate, one branch continuing posteriorly, the other doubling back to lead anteriorly; both form terminal varicose fibers in the circular muscle layer. Nerve lesion experiments, in the adult bullfrog, resulted in accumulations of methionine-enkephalin immunoreactivity at the oral and hinder edges of the laser-irradiated necrotic area; there were sprouting and nonsprouting immunopositive stumps. It is suggested that bidirectional flow of methionine-enkephalin in the myenteric plexus is mediated via the anterior and posterior branches of the axon-like process. The difference in sprouting behavior of immunopositive nerve fiber stumps, after nerve lesion, is discussed with reference to regional differences of the axon-like process.

Neurotensin, substance P, gastrin/cholecystokinin, and bombesin in the intestine of the tilapia (Oreochromis mossambicus) and the goldfish (Carassius auratus): immunochemical detection and effects on electrophysiological characteristics

The distribution of neurotensin-, substance P-, gastrin/cholecystokinin/carerulein- and bombesin-like immunoreactivities has been studied in the gut of the tilapia (Oreochromis mossambicus) and the goldfish (Carassius auratus) using immunohistochemistry and radioimmunoassay; the electrophysiological effects of these peptides on the intestinal epithelium were also examined with the Ussing-type chamber technique. Neurotensin- and gastrin/cholecystokinin/caerulein-like immunoreactivities were present in endocrine cells in both species. Substance P- and bombesin-like immunoreactive endocrine cells were present in the intestine of the tilapia. Neurotensin-like immunoreactivity was observed in varicose fibers and nerve cell bodies in the muscle layers and myenteric plexus of both species, whereas nerve fibers showing substance P-like immunoreactivity were found in the goldfish only. Using radioimmunoassays, neurotensin- and gastrin/cholecystokinin/caerulein-like immunoreactive materials were detected in intestinal extracts of both species. The amounts of substance P- and bombesin-like material were below detection level. The ion selectivity of the intestinal epithelium of both species was modulated by exogenously applied neurotensin. This effect was blocked by tetrodotoxin in the tilapia but not in the goldfish. In the tilapia, neurotensin may act via stimulation of a cAMP-dependent increase of the Cl- conductance of the tight junctions, whereas in the goldfish, neurotensin induced, via an unknown messenger, a transient decrease of the cation selectivity without a decrease in the resistance. Substance P, cholecystokinin, and bombesin were without effect on the electrophysiological characteristics of the epithelium.

Isolation and primary structure of gastrin-releasing peptide from a teleost fish, the trout (Oncorhynchus mykiss)

Immunohistochemical studies have established that fish gastrointestinal tissues contain peptides with gastrin-releasing peptide (GRP)/bombesin-like immunoreactivity, but the molecular nature of this material is unclear. In this study, the most abundant peptide that was immunoreactive towards an antiserum raised against pig GRP was isolated in pure form from an extract of the stomach of the rainbow trout (Oncorhynchus mykiss). The primary structure of the peptide was established as: Ser-Glu-Asn-Thr-Gly-Ala-Ile-Gly-Lys-Val10- Phe-Pro-Arg-Gly-Asn-His-Trp-Ala-Val-Gly20-His-Leu-Met-NH2. Although this amino acid sequence is shorter than those of mammalian GRPs by four residues, the COOH-terminal dodecapeptide is identical to the corresponding region in pig GRP. The data indicate, therefore, that the predominant molecular form of GRP in the stomach of a teleost fish is structurally more similar to mammalian GRP than to the amphibian skin peptide, bombesin.

Substance-P-related and neurokinin-A-related peptides from the brain of the cod and trout

An extract of the brain of the rainbow trout, Oncorhynchus mykiss contained high concentrations of both neurokinin A-like immunoreactivity (corresponding to 90 pmol mammalian neurokinin A/g wet tissue) and substance-P-like immunoreactivity (corresponding to 50 pmol mammalian substance P/g wet tissue) measured by radioimmunoassay using antisera directed against the C-terminal regions of the mammalian peptides. In contrast, an extract of the Atlantic cod. Gadus morhua contained only neurokinin-A-like immunoreactivity (151 pmol/g). This apparent paradox was resolved by determination of the primary structures of the fish tachykinins. Trout substance P (Lys-Pro-Arg-Pro-His-Gln-Phe-Phe-Gly-Leu-MetNH2) has the same amino acid sequence in its C-terminal region as that in the corresponding region of mammalian substance P. Cod substance P (Lys-Pro-Arg-Pro-Gln-Gln-Phe-Ile-Gly-Leu-MetNH2), however, contains a substitution at position 8 (Phe----Ile) that abolishes reactivity with the antiserum to substance P but permits reactivity with the antiserum to neurokinin A. The amino acid sequence of cod and trout neurokinin A is the same (His-Lys-Ile-Asn-Ser-Phe-Val-Gly-Leu-MetNH2) and shows two substitutions (Thr3----Ile and Asp4----Asn) compared with mammalian neurokinin A. The data indicate that nervous tissue of teleost fish contain tachykinins that are analogous to the peptides found in mammalian tissues.

Distributions and colocalization of neuropeptide Y and somatostatin in the goldfish brain

The distributions of single- and double-labelled neuropeptide Y- (NPY-) and somatostatin-immunoreactive (SOM-IR) perikarya and processes were determined in the goldfish brain using immunoperoxidase and immunofluorescence techniques, respectively. In double-labelled material, it was evident that although these two peptides showed markedly similar distributions, they were colocalized in very few instances. A high degree of colocalization of NPY and SOM was noted in the neurons of the ventrolateral telencephalon (VI), the entopenduncular nucleus (NE) and, to a lesser extent, in the dorsocentral nucleus of the telencephalon (Dc). In Vl and NE, neurons showing NPY-IR displayed SOM-IR and vice versa. The only other instance of colocalization was that noted in the brainstem, where SOM and NPY were colocalized in the large cell bodies of the medial column of the vagal motor complex. Single-labelled SOM- and NPY-IR neurons shared a very similar distribution in various nuclei in the diencephalon and in the optic tectum. Colocalization was also noted within fibers throughout many nuclei of the telencephalon and within fibers innervating the swim bladder, one of the peripheral organs to which neurons of the medial column of the vagal motor complex project. Processes in the torus semicircularis and vagal lobe showed single-labelled immunoreactivity for both SOM and NPY in distinct laminar patterns. Large single-labelled SOM-IR terminals appeared to form pericellular baskets in the eminentia granularis of the cerebellum. Single-labelled NPY- or SOM-IR fibers were also found in the secondary gustatory nucleus and tract, the facial lobe, descending trigeminal tract, reticular formation and spinal cord. As in mammalian species, select groups of neurons in teleosts colocalize the neuropeptides SOM and NPY.

Degranulation of eosinophilic granule cells induced by capsaicin and substance P in the intestine of the rainbow trout (Oncorhynchus mykiss Walbaum)

Adult rainbow trout (Oncorhynchus mykiss) were injected intraperitoneally with capsaicin, substance P, serotonin, or a control of saline vehicle or bovine serum albumin (0.5 microgram/g body weight). Fish were sacrificed 30 min and 1, 2, and 4 h post-injection, the gut was dissected out, and a small section of the upper intestine was processed for electron microscopy. A significant proportion of eosinophilic granule cells (EGCs) of the intestine were in close association with non-myelinated neuronal bundles in all fish (4 fish per treatment and time period), but there was no significant difference between treatment or time, suggesting that the association was unaffected by these factors. Close examination of EGC ultrastructure showed that fish treated with capsaicin and substance P exhibited limited degranulation of the EGCs in the stratum compactum and extensive crinophagic-like degranulation in the lamina propria. Cells of the lamina propria contained characteristic multivesicular-like bodies. The degranulation was reminiscent of both mast cell degranulation and endocrine cell crinophagy. EGCs of fish treated with serotonin or a control were unaffected, suggesting that the serotoninergic neurons, believed to be involved in gut motility, were not responsible for degranulation. It is apparent that EGCs of the trout intestine may be under nervous control, as has been demonstrated previously for mammalian mast cells.

Bombesin-like immunoreactivity in skates and the in vitro effect of bombesin on coronary vessels from the longnose skate, Raja rhina

Bombesin-like immunoreactivity is present in nerve fibers projecting to the cardiovascular system, including the coronary arteries, and to the gastrointestinal canal, and in endocrine cells of the gut of skates belonging to the family Rajidae. Synthetic bombesin contracted isolated coronary rings from the longnose skate, Raja rhina, in a cumulative fashion. The contractile response was 84% of that of 60 mM potassium chloride. The pD2-value for bombesin was 8.83 (S.E.M. = 0.33; n = 15). Phentolamine, atropine and two substance P-antagonists increased the sensitivity to bombesin, while atenolol, sotalol, nifedipine, tetrodotoxin and two bombesin antagonists were devoid of significant effects. We conclude from this study that a bombesin-like peptide is present in nerves innervating the cardiovascular system and the gastrointestinal canal of skates of the family Rajidae, and that bombesin contracts coronary vessels in vitro via a direct mechanism and/or via mechanisms involving alpha-adrenergic and muscarinergic receptors.

Tachykinins and intestinal motility in different fish groups

The presence and function of tachykinins were studied in the intestine of hagfish (Myxine glutinosa), lamprey (Lampetra fluviatilis), starry ray (Raja radiata), lungfish (Lepidosiren paradoxa), bichir (Polypterus senegalensis), and rainbow trout (Oncorhynchus mykiss), which represent different systematic groups of fish. Immunohistochemistry revealed cells containing substance P (SP)-like material in the intestine of lamprey and lungfish, and in the stomach of the ray. The intestinal motility was studied using isolated muscle strip preparations. SP had no effect on hagfish or lamprey intestine. In the other four species SP produced intestinal contractions. In ray, bichir, and lungfish the tachykinins may be released from endocrine cells and act, at least in the bichir and lungfish, directly onto the smooth muscle cells. In the rainbow trout intestine, where SP-like material may be released from both nerve fibres and endocrine cells, it is indicated that the contractile effect is in part direct upon the smooth muscle and in part via stimulation of cholinergic and serotonergic neurons.

Effects of vasoactive intestinal polypeptide, substance P, 5-hydroxytryptamine, met-enkephalin and neurotensin on the swimbladder smooth muscle of two teleost species,Gadus morhua andAnguilla anguilla

The effects of vasoactive intestinal polypeptide (VIP), substance P (SP), 5-hydroxytryptamine (5-HT), neurotensin (NT) and met-enkephalin (mEnk) on the smooth muscles of the teleost swimbladder were studied in two teleost species, the Atlantic cod (Gadus morhua) and the eel (Anguilla anguilla). The study was made on isolated strip preparations of the muscularis mucosae, using putative transmitters corresponding to the immunoreactive materials that have previously been localized by immunohistochemical methods in nerves or endocrine/paracrine cells of the teleost swimbladder and/or the gastrointestinal canal. VIP was relaxatory on both cod and eel swimbladder smooth muscle, SP and 5-HT were constrictory in both species, and mEnk was excitatory on the eel swimbladder smooth muscle. Clear effects of these agents were usually seen at a concentration ofca. 10 nM in cumulative concentration/effect experiments. NT had no effect in either species. In the eel, the effects on the pneumatic duct were generally greater than on the swimbladder proper. The study indicates that the 5-HT and peptides previously observed by immunohistochemistry have physiological functions in the swimbladder.

Gut hormones in cyclostomes

The current state of knowledge about regulatory peptides in endocrine cells and nerves of the alimentary canal of lampreys and hagfishes is reviewed. Cyclostomes have a wide range of peptides similar immunochemically to those of higher vertebrates. They include, in the endocrine cells of the intestine, peptides resembling glucagon, gastrin/cholecystokinin, peptide YY (pancreatic polypeptide/neuropeptide Y), substance P, vasoactive intestinal peptide, somatostatin and, in the larval stages at least, insulin. The enteric nerves of some lamprey species contain peptides resembling bombesin (gastrin-releasing peptide) and calcitonin gene-related peptide, as well as serotonin. The occurrence of other peptides is less well documented.Little is known of the molecular structure or the biological roles of the enteric peptides in cyclostomes. Extraction, purification, sequencing and physiological experiments are greatly needed.

Presence of four tachykinins in an acid extract of the carp intestinal bulb (Cyprinus carpio)

1. The pharmacological and chemical properties of substance P-like peptides isolated from an acid extract of the carp intestinal bulb were examined using guinea-pig ileum longitudinal smooth muscle. 2. On a Sephadex G25 column (3 x 96 cm), smooth muscle contracting material was eluted as two peaks (fraction-1 and fraction-2). The molecular weight of the fraction-1 was estimated to be 2300 and that of the fraction-2 to be 1530. 3. The pharmacological properties of the contracting materials in fraction-1 and fraction-2 resembled those of substance P and neurokinin A. 4. The susceptibility of the contracting activity of fraction-1 to proteolytic enzymes resembled that of physalaemin but, on the other hand, the susceptibility of that of fraction-2 resembled those of eledoisin and neurokinin A. 5. Ion-exchange chromatography on sulfopropyl-Sephadex C25 indicated the presence of one contracting material in fraction-1 and three contracting materials in fraction-2. The elution positions of four materials were different from that of substance P. 6. These results indicate that four tachykinins different from substance P are present in an acid extract of the carp intestinal bulb.

Effects of some autonomic drugs and neuropeptides on the mechanical activity of longitudinal and circular muscle strips isolated from the carp intestinal bulb (Cyprinus carpio)

1. The mechanical responses to some autonomic drugs and neuropeptides of longitudinal muscle (LM) and circular muscle (CM) strips isolated from the carp intestinal bulb were investigated in vitro. 2. Acetylcholine and carbamylcholine caused concentration-dependent transient contraction of both LM and CM strips. Tetrodotoxin had no effect, but atropine selectively decreased the contractile responses to acetylcholine and carbamylcholine. 3. Excitatory alpha-2 and inhibitory beta adrenoceptors were present in both LM and CM strips. 4. 5-Hydroxytryptamine (5-HT) caused concentration-dependent contraction of both LM and CM strips. Tetrodotoxin, atropine and methysergide decreased the contractile responses to 5-HT. 5. Some neuropeptides (angiotensin I, angiotensin II, bombesin, bradykinin, neurotensin, somatostatin and vasoactive intestinal polypeptide) did not cause any mechanical response (contraction or relaxation) in either smooth muscle strip. 6. Substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) caused contraction of both LM and CM strips. However, the time course of the contraction in LM was different from that in CM. The order of potency was NKA greater than SP greater than NKB in LM strips and NKA greater than SP much greater than NKB in CM strips. In LM strips, the contractile responses to tachykinins were unaffected by spantide and methysergide, but partly decreased by tetrodotoxin and atropine. On the other hand, the contractile responses of CM strips were unaffected by tetrodotoxin, atropine, methysergide and spantide. 7. Dynorphin (1-13) (DYN), leucine-enkephalin (L-Enk) and methionine-enkephalin (M-Enk) caused concentration-dependent contraction of both LM and CM strips. The order of potency was DYN greater than M-Enk greater than L-Enk. Naloxone selectively decreased the responses to opiate peptides. 8. The present results indicate that acetylcholine, carbamylcholine, catecholamines, 5-HT, tachykinins (SP, NKA and NKB) and opiate peptides (DYN, L-Enk and M-Enk) affect the mechanical activity of LM and CM strips isolated from the carp intestinal bulb through their specific receptors.

5-Hydroxytryptamine is a possible neurotransmitter of the non-cholinergic excitatory nerves in the longitudinal muscle of rainbow trout stomach (Salmo gairdneri)

1. The neurotransmitter of the non-cholinergic excitatory nerves in the rainbow trout stomach was identified on the basis of the pharmacological properties of the contractile responses to transmural stimulation (TMS) and nicotine. 2. TMS caused tetrodotoxin-sensitive contractions of rainbow trout stomach strips in a frequency-dependent manner (0.5-50 Hz). Atropine (1 microM) significantly decreased the contractile response to low-frequency stimulation (0.5-2 Hz), but did not affect that to high-frequency stimulation (3-20 Hz). 3. The atropine-resistant contractile response to TMS (20 Hz) was unaffected by hexamethonium (100 microM), phentolamine (5.4 microM), pyrilamine (1 microM), naloxone (1 microM) or substance P-induced desensitization. 4. 5-Hydroxytryptamine (5-HT, 3 nM-3 microM) caused atropine-resistant contractions in a concentration-dependent manner. In the presence of atropine, methysergide (1 microM) decreased the contractile responses to TMS and 5-HT. 5. Nicotine (3 microM-500 microM) induced atropine-resistant contractions that were completely abolished by tetrodotoxin or hexamethonium. Also methysergide inhibited the contractile responses to nicotine. 6. An acid extract of rainbow trout stomach exhibited atropine-resistant contractions that were decreased by methysergide, in both rainbow trout stomach and guinea-pig ileum longitudinal smooth muscle preparations. 7. The present results indicate that, in longitudinal muscle strips of the rainbow trout stomach, 5-HT is one of the mediators (neurotransmitters) of the non-cholinergic excitatory contractions induced by TMS and nicotine.