Autonomic innervation of the fish gut

Effects of CuSO4 and CuO nanoparticles on fish gut motility in Altantic salmon, Salmo salar

Innovation with metallic nanoparticles (NPs) is increasing, but their effects on the gastrointestinal physiology of fishes is poorly understood. This study explored the effects of CuSO4 and CuO NPs (primary size; 10-20 nm) on gut motility of Salmo salar (Atlantic Salmon), using an isolated gut preparation to record the frequency and amplitude of contractions. The study design used each gut as its own control, with measurements before, during and after exposure to each substance. Nanoparticle tracking analysis (NTA), as expected, showed some particle aggregation of CuO NPs in physiological saline with a hydrodynamic diameter of (mean ± S.E., n = 10) 242 nm ± 42. Exposure to 0.1 mg/L of Cu as CuSO4 displayed no significant effect on both the frequency and amplitude when compared to controls without Cu, and the CuO NP treatment. The CuO NP treatment displayed a similar effect on contractions when compared to controls, where the frequency and amplitude of contractions remained unchanged. However, a significant decrease in number of contractions between pre- and post-exposure phases in the CuSO4 treatment was observed. Impaired contractility remained evident in clean saline after exposure to CuSO4, indicating a continued post-exposure effect. In contrast, guts exposed to CuO NP showed similar numbers of contractions in the exposure and post-exposure phases, indicating no further deterioration. Total Cu concentrations in the tissue as dry weight (dw) were significantly higher in guts exposed to CuO NPs (1060 µg/g dw) than CuSO4 (24 µg/g dw), but CuSO4 was more hazardous with respect to gut motility than the nano form.

Morphological organisation of the digestive tract in the stream catfish Pseudecheneis sulcatus (McClelland)

We describe the histological organisation and mucin content in the digestive tract of the stream catfish Pseudecheneis sulcatus. The aim is to find the modifications of the digestive tract in relation to food resources of its habitat. The oesophageal mucosa consists of stratified squamous epithelium with many mucous-secreting cells. The thick muscularis contains an inner longitudinal and outer circular, striated muscle cells. The stomach is J-shaped and shows 6-7 thick mucosal folds that are separated from the submucosa by an organised muscularis mucosae. The mucosa consists of superficial cells with mucin granules, and deeper simple tubular gastric glands in cardia and fundus, but absent in pyloric region. The glandular epithelium shows oxynticopeptic cells containing zymogen granules and abundant tubulo-vesicular bodies. We provide evidence that the latter arise by budding from smooth endoplasmic reticulum and reach the apical cytoplasm. The anterior intestine shows longer mucosal folds with goblet cells (GC). GC are more in the posterior intestine and highest in the rectum. Myenteric neurons with myelinated and non-myelinated axons innervate the intrinsic musculature from stomach to rectum. Many stem cells are evident in the basal intestinal epithelium. They show darker nuclei and undifferentiated organelles. Mucin histochemistry reveals the predominance of neutral mucin (PAS+ positive) from oesophagus to rectum, and neutral and acidic mucin (alcian blue+, pH 2.5) in the posterior intestine to the rectum, with few GC colocalizing both. Ultrastructural features suggest that the species is adapted to omnivory and this is reflected in the predominance of neutral mucin in the digestive tract.

Immunohistochemical characterisation of the adult Nothobranchius furzeri intestine

Nothobranchius furzeri is emerging as an exciting vertebrate organism in the field of biomedicine, developmental biology and ecotoxicology research. Its short generation time, compressed lifespan and accelerated ageing make it a versatile model for longitudinal studies with high traceability. Although in recent years the use of this model has increased enormously, there is still little information on the anatomy, morphology and histology of its main organs. In this paper, we present a description of the digestive system of N. furzeri, with emphasis on the intestine. We note that the general architecture of the intestinal tissue is shared with other vertebrates, and includes a folding mucosa, an outer muscle layer and a myenteric plexus. By immunohistochemical analysis, we reveal that the mucosa harbours the same type of epithelial cells observed in mammals, including enterocytes, goblet cells and enteroendocrine cells, and that the myenteric neurons express neurotransmitters common to other species, such as serotonin, substance P and tyrosine hydroxylase. In addition, we detect the presence of a proliferative compartment at the base of the intestinal folds. The description of the normal intestinal morphology provided here constitutes a baseline information to contrast with tissue alterations in future lines of research assessing pathologies, ageing-related diseases or damage caused by toxic agents.

hapln1a+ cells guide coronary growth during heart morphogenesis and regeneration

Although several tissues and chemokines orchestrate coronary formation, the guidance cues for coronary growth remain unclear. Here, we profile the juvenile zebrafish epicardium during coronary vascularization and identify hapln1a+ cells enriched with vascular-regulating genes. hapln1a+ cells not only envelop vessels but also form linear structures ahead of coronary sprouts. Live-imaging demonstrates that coronary growth occurs along these pre-formed structures, with depletion of hapln1a+ cells blocking this growth. hapln1a+ cells also pre-lead coronary sprouts during regeneration and hapln1a+ cell loss inhibits revascularization. Further, we identify serpine1 expression in hapln1a+ cells adjacent to coronary sprouts, and serpine1 inhibition blocks vascularization and revascularization. Moreover, we observe the hapln1a substrate, hyaluronan, forming linear structures along and preceding coronary vessels. Depletion of hapln1a+ cells or serpine1 activity inhibition disrupts hyaluronan structure. Our studies reveal that hapln1a+ cells and serpine1 are required for coronary production by establishing a microenvironment to facilitate guided coronary growth.

Methodological considerations in studying digestive system physiology in octopus: limitations, lacunae and lessons learnt

Current understanding of cephalopod digestive tract physiology is based on relatively “old” literature and a “mosaic of data” from multiple species. To provide a background to the discussion of methodologies for investigating physiology we first review the anatomy of the cephalopod digestive tract with a focus on Octopus vulgaris, highlighting structure-function relationships and species differences with potential functional consequences (e.g., absence of a crop in cuttlefish and squid; presence of a caecal sac in squid). We caution about extrapolation of data on the digestive system physiology from one cephalopod species to another because of the anatomical differences. The contribution of anatomical and histological techniques (e.g., digestive enzyme histochemistry and neurotransmitter immunohistochemistry) to understanding physiological processes is discussed. For each major digestive tract function we briefly review current knowledge, and then discuss techniques and their limitations for the following parameters: 1) Measuring motility in vitro (e.g., spatiotemporal mapping, tension and pressure), in vivo (labelled food, high resolution ultrasound) and aspects of pharmacology; 2) Measuring food ingestion and the time course of digestion with an emphasis on understanding enzyme function in each gut region with respect to time; 3) Assessing transepithelial transport of nutrients; 4) Measuring the energetic cost of food processing, impact of environmental temperature and metabolic rate (flow-through/intermittent respirometry); 4) Investigating neural (brain, gastric ganglion, enteric) and endocrine control processes with an emphasis on application of molecular techniques to identify receptors and their ligands. A number of major knowledge lacunae are identified where available techniques need to be applied to cephalopods, these include: 1) What is the physiological function of the caecal leaflets and intestinal typhlosoles in octopus? 2) What role does the transepithelial transport in the caecum and intestine play in ion, water and nutrient transport? 3) What information is signalled from the digestive tract to the brain regarding the food ingested and the progress of digestion? It is hoped that by combining discussion of the physiology of the cephalopod digestive system with an overview of techniques and identification of key knowledge gaps that this will encourage a more systematic approach to research in this area.

Effect of partial substitution of fishmeal with insect meal (Hermetia illucens) on gut neuromuscular function in Gilthead sea bream (Sparus aurata)

Alternative nutrient sources to fishmeal for fish feed, such as insect meals, represent a promising sustainable supply. However, the consequences for fish digestive function have not been exhaustively investigated. In the present study we evaluated the effect of partial fishmeal substitution with 10% Hermetia illucens (Hi10) larvae meal on the neuromuscular function of proximal and distal intestine in gilthead sea bream. In animals fed with insect meal, weight and growth parameters were similar to controls fed with conventional fishmeal. In addition, no anomalies in intestinal gross morphology and no overt signs of inflammation were observed. The gastrointestinal transit was significantly reduced in Hi10 fed animals. In the proximal and distal intestine longitudinal muscle, Hi10 feeding downregulated the excitatory cholinergic and serotoninergic transmission. Sodium nitroprusside-induced inhibitory relaxations increased in the proximal intestine and decreased in the distal intestine after Hi10 meal. Changes in the excitatory and inhibitory components of peristalsis were associated with adaptive changes in the chemical coding of both proximal and distal intestine myenteric plexus. However, these neuromuscular function alterations were not associated with considerable variations in morphometric growth parameters, suggesting that 10% Hi meal may represent a tolerable alternative protein source for gilthead sea bream diets.

Acid-Sensing Ion Channels in Zebrafish

Simple Summary

The present review collects data regarding the presence of ASICs (acid-sensing ion channels) in zebrafish, which have become, over several years, an important experimental model for the study of various diseases. ASICs are a family of ion channels involved in the perception of different types of stimuli. They are excitatory receptors for extracellular H⁺ involved in synaptic transmission, the peripheral perception of pain and in chemical or mechanosensation.

Abstract

The ASICs, in mammals as in fish, control deviations from the physiological values of extracellular pH, and are involved in mechanoreception, nociception, or taste receptions. They are widely expressed in the central and peripheral nervous system. In this review, we summarized the data about the presence and localization of ASICs in different organs of zebrafish that represent one of the most used experimental models for the study of several diseases. In particular, we analyzed the data obtained by immunohistochemical and molecular biology techniques concerning the presence and expression of ASICs in the sensory organs, such as the olfactory rosette, lateral line, inner ear, taste buds, and in the gut and brain of zebrafish.

Dietary inclusion of plant ingredients induces epigenetic changes in the intestine of zebrafish

Epigenetic modifications, such as DNA methylation, can be regulated by nutrition and dietary factors. There has been a large increase in the use of sustainable plant-based protein sources in fish feed due to limitations of fishmeal resources, which are needed to sustain a rapidly growing aquaculture industry. With this major transition from marine ingredients to plant-based diets, fish are abruptly introduced to changes in dietary composition and exposed to a variety of phytochemicals, some of which known to cause epigenetic changes in mammals. However, the effect of plant ingredients on the epigenome of fish is barely understood. In the present study, the nutriepigenomic effects of the addition of pea, soy, and wheat gluten protein concentrate to aquafeeds were investigated using zebrafish as a model. A genome-wide analysis of DNA methylation patterns was performed by reduced representation bisulphite sequencing to examine global epigenetic alterations in the mid intestine after a 42-day feeding trial. We found that inclusion of 30% of wheat gluten, pea and soy protein concentrate in the diet induced epigenetic changes in the mid intestine of zebrafish. A large number of genes and intergenic regions were differentially methylated with plant-based diets. The genes concerned were related to immunity, NF-κB system, ubiquitin-proteasome pathway, MAPK pathway, and the antioxidant defence system. Epigenetic regulation of several biological processes, including neurogenesis, cell adhesion, response to stress and immunity was also observed. Ultimately, the observed epigenetic changes may enable zebrafish to rapidly regulate inflammation and maintain intestinal homoeostasis when fed plant protein-based diets.

Distribution of goblet and endocrine cells in the intestine: A comparative study in Amazonian freshwater Tambaqui and hybrid catfish

Goblet cells (GCs) and endocrine cells (ECs) play an important role in intestine physiology, and few studies currently exist for Amazonian fishes. This study aimed to quantify the distribution of GCs and ECs producing cholecystokinin-8 and neuropeptide Y, assessed by mucin histochemistry and peptides immunohistochemistry, in the intestine of two Amazonian species with different feeding habits Tambaqui (Colossosoma macropomum) and hybrid catfish (Pseudoplatystoma reticulatum × Leiarius marmoratus), an omnivore and carnivore, respectively. A systematic literature review correlating feeding habit and GC and EC distribution was also included to contribute to the comparative study. The results of this study provided novel information about the gut cells of Tambaqui and hybrid catfish. Both, GCs and ECs can be found sweeping the entire intestine of Tambaqui and hybrid catfish although the cells can be more concentrated in certain segments. The GCs and ECs in Tambaqui were more uniformly distributed in the midgut segments (T1, T2, and T3). Unlike, in hybrid catfish GCs were more concentrated in the hindgut (C4) and ECs mainly in the two midgut segments (C1 and C2) of hybrid catfish. Based on the comparison between Tambaqui, hybrid catfish, and other fishes in the literature review, we suggest that cell distribution can be partially explained by feeding habits, carnivorous vs. omnivorous.

Adaptative responses of myenteric neurons of Sphoeroides testudineus to environmental pollution

Contamination in estuarine regions affects the local biota damaging the ecosystems and reaching humans. The gastrointestinal tract is a dynamic environment capable of obtaining nutrients and energy from food while it protects the host against harmful toxins and pathogens from the external environment. These functions are modulated by the enteric nervous system and changes in its structure can result in gastrointestinal disorders. The objective of this study was to evaluate if the environmental contaminants have effects on the myenteric neuronal plasticity of pufferfish Sphoeroides testudineus. Animals were collected in Barra do Una River, located at Jureia-Itatins Mosaic of Protected Areas (reference area - RA) and in the Santos Estuarine System (impacted area - IA). Morpho-quantitative analyses of the general and metabolically active myenteric neuronal populations of the proximal and distal intestine were made. Disarrangement was observed in the general organization of the myenteric plexus, with an expressive reduction of the neuronal groups (nodes) in the animals of IA. The vulnerability of the myenteric plexus was evidenced by a decrease in density and cellular profile of the general neuronal population, followed by an increase of the metabolism of the remaining neurons, which in turn was verified by a growth of the area of the cellular and nuclear profiles of the metabolically active neuronal population. Through these analyses, we concluded that animals inhabiting polluted regions present alterations in the myenteric neuronal plasticity, as a way of maintaining the functions of the gastrointestinal tract.

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.

Endocrine cells producing peptide hormones in the intestine of Nile tilapia: distribution and effects of feeding and fasting on the cell density

Endocrine cells (ECs) act as a luminal surveillance system responding to either the presence or absence of food in the gut through the secretion of peptide hormones. The aim of this study was to analyze the effects of feeding and fasting on the EC peptide-specific distribution along the intestine of Nile tilapia. We assessed the density of ECs producing gastrin (GAS), cholecystokinin-8 (CCK-8), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) in nine segments of the intestine using immunohistochemistry. Our results show that ECs immunoreactive to CCK-8, GAS, NPY, and CGRP can be found along all the intestinal segments sampled, from the midgut to hindgut, although differences in their distribution along the gut were observed. Regarding nutrient status, we found that the anterior segments of the midgut seem to be the main site responding to luminal changes in Nile tilapia. The NPY+ and CGRP+ EC densities increased in the fasted group, while the amount of CCK-8+ ECs were higher in the fed group. No effects of fasting or feeding were found in the GAS+ EC densities. Changes in ECs density were found only at the anterior segments of the intestine which may be due to the correlation between vagus nerve anatomy, EC location, and peptide turnover. Lastly, ECs may need to be considered an active cell subpopulation that may adapt and respond to different nutrient status as stimuli. Due to the complexity of the enteroendocrine system and its importance in fish nutrition, much remains to be elucidated and it deserves closer attention.

Orexins and receptor OX2R in the gastroenteric apparatus of two teleostean species: Dicentrarchus labrax and Carassius auratus

Orexin A and B peptides and the receptor OX2R were studied in sea bass and goldfish gastroenteric tract by immunoblotting combined with densitometric analysis using NIH Image J software and immunohistochemical techniques. These teleost species present a different gut organization and diverse feeding habits. Immunoblotting experiments showed one band of 16 kDa corresponding to prepro-orexin, and one band of 38 kDa corresponding to the OX2R receptor. Immunohistochemical localization of OXA and OXB was observed in the enteric nervous system throughout the gastroenteric tract of both species. OXA and OXB immunoreactive cells were found in the gastric and intestinal regions of sea bass, and were mainly found in the basal region of folds in intestinal bulb, and in the midgut and hindgut of goldfish. The distribution of OX2R was mainly detected in the mucosa of the gastroenteric tract of sea bass and goldfish. This distribution suggests an endocrine action of OXA and OXB in the gastrointestinal tract as well as involvement in the peripheral control of food intake and digestive processes in both species. This study might also serve to determine the productive factors in breeding and as a baseline for future experimental studies on the regulation of the gastroenteric functions in non-mammalian vertebrates. Anat Rec, 299:1121-1129, 2016. © 2016 Wiley Periodicals, Inc.

Intrinsic and extrinsic innervation of the heart in zebrafish (Danio rerio)

In the vertebrate heart the intracardiac nervous system is the final common pathway for autonomic control of cardiac output, but the neuroanatomy of this system is not well understood. In this study we investigated the innervation of the heart in a model vertebrate, the zebrafish. We used antibodies against acetylated tubulin, human neuronal protein C/D, choline acetyltransferase, tyrosine hydroxylase, neuronal nitric oxide synthase, and vasoactive intestinal polypeptide to visualize neural elements and their neurotransmitter content. Most neurons were located at the venous pole in a plexus around the sinoatrial valve; mean total number of cells was 197 ± 23, and 92% were choline acetyltransferase positive, implying a cholinergic role. The plexus contained cholinergic, adrenergic, and nitrergic axons and vasoactive intestinal polypeptide-positive terminals, some innervating somata. Putative pacemaker cells near the plexus showed immunoreactivity for hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) and the transcription factor Islet-1 (Isl1). The neurotracer neurobiotin showed that extrinsic axons from the left and right vagosympathetic trunks innervated the sinoatrial plexus proximal to their entry into the heart; some extrinsic axons from each trunk also projected into the medial dorsal plexus region. Extrinsic axons also innervated the atrial and ventricular walls. An extracardiac nerve trunk innervated the bulbus arteriosus and entered the arterial pole of the heart to innervate the proximal ventricle. We have shown that the intracardiac nervous system in the zebrafish is anatomically and neurochemically complex, providing a substrate for autonomic control of cardiac effectors in all chambers.

Regional innervation of the heart in the goldfish, Carassius auratus: a confocal microscopy study

The intracardiac nervous system represents the final common pathway for autonomic control of the vertebrate heart in maintaining cardiovascular homeostasis. In teleost fishes, details of the organization of this system are not well understood. Here we investigated innervation patterns in the heart of the goldfish, a species representative of a large group of cyprinids. We used antibodies against the neuronal markers zn-12, acetylated tubulin, and human neuronal protein C/D, as well as choline acetyltransferase, tyrosine hydroxylase, nitric oxide synthetase, and vasoactive intestinal polypeptide (VIP) to detect neural elements and their transmitter contents in wholemounts and sections of cardiac tissue. All chambers of the heart were innervated by choline acetyltransferase-positive axons, implying cholinergic regulation; and by tyrosine hydroxylase-containing axons, implying adrenergic regulation. The mean total number of intracardiac neurons was 713 ± 78 (SE), nearly half of which were cholinergic. Neuronal somata were mainly located in a ganglionated plexus around the sinoatrial valves. Somata were contacted by cholinergic, adrenergic, nitrergic, and VIP-positive terminals. Putative pacemaker cells, identified by immunoreactivity for hyperpolarization activated, cyclic nucleotide-gated channel 4, were located in the base of the sinoatrial valves, and this region was densely innervated by cholinergic and adrenergic terminals. We have shown that the goldfish heart possesses the necessary neuroanatomical substrate for fine, region-by-region autonomic control of the myocardial effectors that are involved in determining cardiac output.

Proteinase-activated receptor 2 expression in the intestinal tract of the horse

Proteinase-activated receptor 2 (PAR2) is a G-protein-coupled receptor for trypsin and mast cell tryptase; it is highly expressed at the intestinal level with multiple functions, such as epithelial permeability and intestinal motility. Many proteases activate PAR2 during tissue damage, suggesting a role of the inflammatory response receptors. The aim of the study was to evaluate the distribution and expression of PAR2 in the jejunum, the ileum and the pelvic flexure, using samples collected from healthy adult horses after slaughter. Proteinase-activated receptor 2 immunoreactivity (PAR2-IR) was observed in the enterocytes, intestinal glands, the smooth muscle of the muscularis mucosae, and the longitudinal and circular muscle layers; there were no differences in the distribution of PAR2-IR in the different sections of the intestinal tract. The protein expression level showed that the relative amount of the PAR2 content in the mucosa of the intestinal tract decreased from the small to the large intestine while the PAR2 mRNA analysed showed similar values. This study provides relevant findings concerning the distribution of the PAR2 in the intestines of healthy horses and represents the starting point for evaluating the role of the PAR2 during strangulative intestinal disease and consequent systemic intestinal reperfusion/injury complications in horses in order to identify and employ antagonist PAR2 molecules.

Enteric neuroplasticity in seawater-adapted European eel (Anguilla anguilla)

European eels live most of their lives in freshwater until spawning migration to the Sargasso Sea. During seawater adaptation, eels modify their physiology, and their digestive system adapts to the new environment, drinking salt water to compensate for the continuous water loss. In that period, eels stop feeding until spawning. Thus, the eel represents a unique model to understand the adaptive changes of the enteric nervous system (ENS) to modified salinity and starvation. To this purpose, we assessed and compared the enteric neuronal density in the cranial portion of the intestine of freshwater eels (control), lagoon eels captured in brackish water before their migration to the Sargasso Sea (T0), and starved seawater eels hormonally induced to sexual maturity (T18; 18 weeks of starvation and treatment with standardized carp pituitary extract). Furthermore, we analyzed the modification of intestinal neuronal density of hormonally untreated eels during prolonged starvation (10 weeks) in seawater and freshwater. The density of myenteric (MP) and submucosal plexus (SMP) HuC/D-immunoreactive (Hu-IR) neurons was assessed in wholemount preparations and cryosections. The number of MP and SMP HuC/D-IR neurons progressively increased from the freshwater to the salty water habitat (control > T0 > T18; P < 0.05). Compared with freshwater eels, the number of MP and SMP HuC/D-IR neurons significantly increased (P < 0.05) in the intestine of starved untreated salt water eels. In conclusion, high salinity evokes enteric neuroplasticity as indicated by the increasing number of HuC/D-IR MP and SMP neurons, a mechanism likely contributing to maintaining the body homeostasis of this fish in extreme conditions.

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.

Artificial selection on relative brain size in the guppy reveals costs and benefits of evolving a larger brain

The large variation in brain size that exists in the animal kingdom has been suggested to have evolved through the balance between selective advantages of greater cognitive ability and the prohibitively high energy demands of a larger brain (the “expensive-tissue hypothesis” [1]). Despite over a century of research on the evolution of brain size, empirical support for the trade-off between cognitive ability and energetic costs is based exclusively on correlative evidence [2], and the theory remains controversial [3, 4]. Here we provide experimental evidence for costs and benefits of increased brain size. We used artificial selection for large and small brain size relative to body size in a live-bearing fish, the guppy (Poecilia reticulata), and found that relative brain size evolved rapidly in response to divergent selection in both sexes. Large-brained females outperformed small-brained females in a numerical learning assay designed to test cognitive ability. Moreover, large-brained lines, especially males, developed smaller guts, as predicted by the expensive-tissue hypothesis [1], and produced fewer offspring. We propose that the evolution of brain size is mediated by a functional trade-off between increased cognitive ability and reproductive performance and discuss the implications of these findings for vertebrate brain evolution.

Complex innervation patterns of the conus arteriosus in the heart of the longnose gar, Lepisosteus osseus

Anatomical and functional studies of the autonomic innervation in the conus arteriosus of the garfishes are lacking. This study reveals that the conus arteriosus of the longnose gar is primarily myocardial in nature, but additionally, large numbers of smooth muscle cells are present in the subendocardium. A well-developed system of adrenergic, cholinergic, substance P (SP) and neuronal nitric oxide synthase (nNOS) positive nerve terminals are found in the wall of the conus arteriosus. Coronary blood vessels running in the adventitia receive a rich supply of nNOS positive nerve fibers, thus suggesting their importance in the nitrergic control of blood flow in the conus arteriosus. The present data show that the patterns of autonomic innervation of the garfish conus arteriosus are more complex than previously appreciated.

Immunolocalization of neurotransmitter-synthesizing enzymes and neuropeptides with associated receptors in the photophores of the hatchetfish, Argyropelecus hemigymnus Cocco, 1829

Anatomical and functional studies of the autonomic innervation of the photophores of luminescent fishes are scarce. The present immunohistochemical study demonstrated the presence of nerve fibers in the luminous epithelium and lens epithelium of the photophores of the hatchet fish, Argyropelecus hemigymnus and identified the immunoreactive elements of this innervation. Phenylethanolanine N-methyltransferase (PNMT) and catecholamine (CA)-synthesizing enzymes were detected in nerve varicosities inside the two epithelia. Neuropeptides were localized in neuropeptide Y (NPY) and substance P (SP)- and its NK11 receptor-immunopositive nerves in the lens epithelium. Neuropeptides were also localized in non-neural cell types such as the lens cells, which displayed immunoreactivities for pituitary adenylate cyclase activating peptide (PACAP) and their receptors R-12 and 93093-3. This reflects the ability of the neuropeptide-containing nerves and lens cells to turn on and off the expression of selected messengers. It appears that the neuropeptide-containing nerves demonstrated in this study may be sensory. Furthermore, neuronal nitric oxide synthase-immunopositive axons associated with photocytes in the luminous epithelium have previously been described in this species. Whereas it is clear that the photophores receive efferent (motor) fibers of spinal sympathetic origin, the origin of the neuropeptide sensory innervation remains to be determined. The functional roles of the above neuropeptides or their effects on the bioluminescence or the chemical nature of the terminals, either sensory or postganglionic neurons innervating the photophores, are still not known.

Melatonin effects on gut motility are independent of the relaxation mediated by the nitrergic system in the goldfish

Melatonin is a key neuroendocrine transducer in the circadian organization of vertebrates. However, its role in gastrointestinal physiology has not been explored in depth. In goldfish, a role for melatonin as a modulator of intestinal motility has been reported, whereby it attenuates the cholinergic contraction. The aim of the present work was to investigate this relaxation induced by melatonin in the gut smooth muscle of the goldfish, studying the possible involvement of nitric oxide. An in vitro model of isolated goldfish intestine was used to test the effects on intestinal motility. The addition of melatonin (10 pM-100 μM) to the organ bath relaxed acetylcholine- and serotonin-stimulated gut strips, but no effect was observed on KCl-contracted preparations. The addition of L-NAME (nitric oxide synthase inhibitor) increased the amplitude of the spontaneous slow waves, while sodium nitroprusside (SNP, nitric oxide donor) abolished them. All these results support a role for the nitrergic system in goldfish gut motility. However, neither L-NAME, nor SNP nor the nitric oxide precursor, l-arginine, modified the melatonin relaxing effect. These results highlight the existence of a basal nitrergic tone in the gut of goldfish, where melatonin would exert a calcium-dependent, nitric oxide-independent relaxing effect on serotonergic and cholinergic contraction.

Neurochemical coding of enteric neurons in adult and embryonic zebrafish (Danio rerio)

Although the morphology and development of the zebrafish enteric nervous system have been extensively studied, the precise neurochemical coding of enteric neurons and their proportional enteric distribution are currently not known. By using immunohistochemistry, we determined the proportional expression and coexpression of neurochemical markers in the embryonic and adult zebrafish intestine. Tyrosine hydroxylase (TH), vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating peptide (PACAP) were observed only in nerve fibers, whereas other markers were also detected in neuronal cell bodies. Calretinin and calbindin had similar distributions. In embryos, all markers, except for choline acetyltransferase (ChAT) and TH, were present from 72 hours postfertilization. Nitrergic neurons, evenly distributed and remaining constant in time, constituted the major neuronal subpopulation. The neuronal proportions of the other markers increased during development and were characterized by regional differences. In the adult, all markers examined were expressed in the enteric nervous system. A large percentage of enteric neurons displayed calbindin and calretinin, and serotonin was the only marker showing significant distribution differences in the three intestinal regions. Colocalization studies showed that serotonin was not coexpressed with any of the other markers. At least five neuronal subpopulations were determined: a serotonergic, a nitrergic noncholinergic, two cholinergic nonnitrergic subpopulations along with one subpopulation expressing both ChAT and neuronal nitric oxide synthase. Analysis of nerve fibers revealed that nitrergic neurons coexpress VIP and PACAP, and that nitrergic neurons innervate the tunica muscularis, whereas serotonergic and cholinergic nonnitrergic neurons innervate the lamina propria and the tunica muscularis.

Serotonin-induced contraction in isolated intestine from a teleost fish (Carassius auratus): characterization and interactions with melatonin

BACKGROUND

Serotonin (5-HT) plays a critical role in several gastrointestinal functions in vertebrates. In teleosts lacking enterochromaffin cells, intestinal 5-HT originates from serotonergic enteric neurons. In the present study, the foregut of a stomachless teleost, the goldfish (Carassius auratus), was used to evaluate the in vitro effect of 5-HT on fish intestinal motility. We also studied the role of melatonin (MEL), an indoleamine sharing the biosynthetic pathway with 5-HT, as regulator of serotonergic activity.

METHODS

An organ bath system, with longitudinal strips from the goldfish intestinal bulb attached to an isometric transducer was used to record foregut smooth muscle contractions.

KEY RESULTS

Concentration-dependent curves of the contractile response exerted by 5-HT and its agonists, 5-methoxytryptamine (5-MT) and 5-carboxamidotryptamine (5-CT), suggest a receptor-mediated action, supported by the blockade by a general 5-HT antagonist, methysergide. The 5-HT-induced contraction was abolished in the presence of atropine, revealing the involvement of cholinergic transmission in gut actions of 5-HT. Furthermore, MEL inhibited the contractile effect of 5-HT and its agonists by up to 50%, which was counteracted by MEL antagonists.

CONCLUSIONS & INFERENCES

We can provisionally propose that at least two different 5-HT receptor subtypes are involved in fish intestinal motility, a 5-HT₄-like (5-MT-preferring) and a 5-HT₇-like (5-CT- and fluphenazine-sensitive) receptor. In summary, our results indicate that 5-HT regulates the contractile activity of goldfish foregut through specific receptors located in cholinergic neurons, and that MEL can modulate these serotonergic actions through high-affinity membrane receptors.

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.

Gut glucose metabolism in rainbow trout: implications in glucose homeostasis and glucosensing capacity

The main objective of the present study was to evaluate the relative contribution of the intestine to glucose homeostasis in rainbow trout. In a first set of in vivo experiments trout were subjected to oral glucose treatments alone or in combination with insulin injections to assess changes in glucose-related enzymes activities, metabolite levels, and mRNA levels. Rainbow trout gut displays an important glucose metabolism that includes the ability to store glucose as glycogen (mostly in the muscle layers) and a large capacity to oxidize glucose. This constitutes a surprising result for a carnivorous fish. In a second set of in vivo experiments, trout received an oral amino acid solution alone or in combination with insulin injection to determine whether other factors besides fasting could regulate gluconeogenesis in intestine. The results confirm the absence of regulation of gluconeogenesis in trout gut, which does not respond to hormones, glucose, lactate, or amino acid changes, either in vivo or in vitro. We also fully characterized gut glucose metabolism in vitro. We observed that a large amount of glucose is oxidized to lactate, supporting the importance of glucose in gut metabolism. Moreover, we corroborated the minor actions of insulin in trout gut, whereas other hormones such as glucagon-like peptide-1 and C-peptide appear to be major hormonal regulators of glucose metabolism in fish gut. Finally, we obtained the first evidence for the existence of a glucosensing mechanism in the midgut of this carnivorous species.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.