Enteric reflexes and nitric oxide in the fish intestine

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.

Identification of neurogenic intestinal motility patterns in silver perch (Bidyanus bidyanus) that persist over wide temperature ranges

BACKGROUND

Fish are increasingly being utilized as a model species for genetic manipulation studies related to gastrointestinal (GI) motility. Our aim was to identify whether patterns of GI motility in fish and the mechanisms underlying their generation are similar to those recorded from mammals (including humans).

METHODS

The entire intestine was removed from euthanized adult Silver Perch (n = 11) and lesioned at the midway point to obtain two equal lengths. Proximal and distal segments were studied separately in organ baths with oxygenated Krebs solution, maintained at either 15°C (n = 5) or 25°C (n = 6). Motility was analyzed during rest, after oral infusion of Krebs solution, and after application of hexamethonium (100 µM) and tetrodotoxin (TTX) (0.6 µM).

KEY RESULTS

Antegrade and retrograde propagating contractions (PC) were recorded in all preparations. In the proximal intestine, at 15 and 25°C, retrograde PCs occurred at 2.7 [1.7-4.5] and 3.1 [1.6-6.5] times the frequency of antegrade PCs, respectively. Colder temperatures did not inhibit PC frequency. Hexamethonium did not inhibit PC, and however, TTX abolished all contractile activity.

CONCLUSIONS AND INFERENCES

Both neurogenic antegrade and retrograde propagating contractions occur throughout the intestine of Silver Perch. However, unlike the mammalian colon, these motor patterns do not require enteric nicotinic transmission and they are not inhibited by cold temperatures (15°C). Therefore, while the GI motility patterns in Silver Perch resemble those recorded from the colon of mammals, there may be differences in the mechanisms that underlying their generation.

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.

Rheological characterization of an in vitro model for salmonid chyme to quantify changes in feed composition

BACKGROUND

Developments in the production of aquacultural salmonid feeds in the last 20 years have led to extruded diets with extremely low water content and a shift from mainly marine fish based ingredients towards plant content. These changes expose the industry to the vagaries of the highly dynamic plant protein market. Resulting variations in the precise composition of aqua feeds may carry unpredictable consequences for water quality, since some plant ingredients cause undesirable reductions in the mechanical stability of faeces. Dietary supplements known as binders that enhance the stability of faeces have the potential to mitigate these issues, but may also bring negative effects.

OBJECTIVE AND METHODS

The present study employs an in vitro model to perform the first fundamental rheological characterization of salmonid chyme, and a factorial experiment designed to investigate the impacts of the presence of rheologically active substances.

RESULTS

The highest mean viscosity values were measured for a treatment containing a 2:1 ratio of tara gum:xanthan gum, resulting in chyme four times more viscous than a control formulation containing the same amount of water. Shear resistance was quantified by analyses of slopes fitting the frequency sweep measurements.

CONCLUSIONS

These data open a new statistical approach to monitoring the consequences of market-driven changes in aqua feed composition and their impacts on the nutrition, health or performance of farmed fish.

Dynamic changes in nitric oxide synthase expression are involved in seawater acclimation of rainbow trout Oncorhynchus mykiss

Recent research has shown that nitric oxide (NO) produced by nitric oxide synthases (NOS) is an inhibitor of ion transporter activity and a modulator of epithelial ion transport in fish, but little is known on changes in the NOS/NO system during osmotic stress. We hypothesized that the NOS/NO system responds to salinity changes as an integrated part of the acclimation process. Expression and localization of nos1/Nos1 and nos2/Nos2 were investigated in gill, kidney, and intestine of freshwater (FW)- and seawater (SW)-transferred trout using quantitative PCR, Western blotting, and immunohistochemistry, along with expressional changes of major ion transporters in the gill. The classical branchial ion transporters showed expected expressional changes upon SW transfer, there among a rapid decrease in Slc26a6 mRNA, coding a branchial Cl^-/[Formula: see text] exchanger. There was a major downregulation of nos1/ nos2/Nos2 expression in the gill during SW acclimation. A significant decrease in plasma nitrite supported an overall decreased Nos activity and NO production. In the middle intestine, Nos1 was upregulated during SW acclimation, whereas no changes in nos/Nos expression were observed in the posterior intestine and the kidney. Nos1 was localized along the longitudinal axis of the gill filament, beneath smooth muscle fibers of the intestine wall and in blood vessel walls of the kidney. Nos2 was localized within the epithelium adjacent to the gill filament axis and in hematopoietic tissues of the kidney. We conclude that downregulation of branchial NOS is integrated to the SW acclimation process likely to avoid the inhibitory effects of NO on active ion extrusion.

Cortisol regulates nitric oxide synthase in freshwater and seawater acclimated rainbow trout, Oncorhynchus mykiss

Cortisol and nitric oxide (NO) are regulators of ion transport and metabolic functions in fish. In the gill, they show opposite effects on Na⁺/K⁺-ATPase (NKA) activity: cortisol stimulates NKA activity while NO inhibits NKA activity. We hypothesized that cortisol may impact NO production in osmoregulatory tissues by regulating NO synthase (NOS) expression. We evaluated the influence of cortisol treatment on mRNA expression of Nos1 and Nos2 in gill, kidney and middle intestine of both freshwater (FW) and seawater (SW) acclimated rainbow trout and found both tissue- and salinity-dependent effects. Nos2 expression was down-regulated in the gill by cortisol injection in both FW and SW trout. This was substantiated by incubating gill tissue with cortisol ex vivo. Similarly, cortisol injection significantly down-regulated Nos2 expression in kidney of SW fish but not in FW fish. In the middle intestine, Nos2 expression was up-regulated by cortisol injection in FW but unchanged in SW fish. Nos1 expression was up-regulated by cortisol injection in FW kidney and down-regulated in SW kidney, whereas it was unaffected in gill and middle intestine of FW and SW fish. Our data provide the first evidence that cortisol may influence NO production in fish by regulating Nos expression. Indeed, the down-regulation of Nos2 expression by cortisol in the gill may prevent the inhibitory effect of NO on NKA activity thereby furthering the stimulatory effect of cortisol on ion-transport.

The presence and role of interstitial cells of Cajal in the proximal intestine of shorthorn sculpin (Myoxocephalus scorpius)

Rhythmic contractions of the mammalian gastrointestinal tract can occur in the absence of neuronal or hormonal stimulation owing to the generation of spontaneous electrical activity by interstitial cells of Cajal (ICC) that are electrically coupled to smooth muscle cells. The myogenically driven component of gastrointestinal motility patterns in fish probably also involves ICC; however, little is known of their presence, distribution and function in any fish species. In the present study, we combined immunohistochemistry and in vivo recordings of intestinal motility to investigate the involvement of ICC in the motility of the proximal intestine in adult shorthorn sculpin (Myoxocephalus scorpius). Antibodies against anoctamin 1 (Ano1, a Ca²⁺-activated Cl^- channel), revealed a dense network of multipolar, repeatedly branching cells in the myenteric region of the proximal intestine, similar in many regards to the mammalian ICC-MY network. The addition of benzbromarone, a potent blocker of Ano1, altered the motility patterns seen in vivo after neural blockade with TTX. The results indicate that ICC are integral for the generation and propagation of the majority of rhythmic contractile patterns in fish, although their frequency and amplitude can be modulated via neural activity.

Effects of feeding on in vivo motility patterns in the proximal intestine of shorthorn sculpin (Myoxocephalus scorpius)

This is the first study to catalogue the diverse array of in vivo motility patterns in a teleost fish and how they are affected by feeding. Video recordings of exteriorised proximal intestine from fasted and fed shorthorn sculpin (Myoxocephalus scorpius) were used to generate spatio-temporal maps to portray and quantify motility patterns. Propagating and non-propagating contractions were observed to occur at different frequencies and durations. The most apparent difference between the feeding states was that bands of relatively high amplitude contractions propagating slowly in the anal direction were observed in all fasted fish (N=10) but in only 35% of fed fish (N=11). Additionally, fed fish displayed a reduced frequency (0.21±0.03 versus 0.32±0.06 contractions min(-1)) and rhythmicity of these contractions compared with fasted fish. Although the underlying mechanisms of these slow anally propagating contractions differ from those of mammalian migrating motor complexes, we believe that they may play a similar role in shorthorn sculpin during the interdigestive period, to potentially remove food remnants and prevent the establishment of pathogens. 'Ripples' were the most prevalent contraction type in shorthorn sculpin and may be important during mixing and absorption. The persistence of shallow ripples and pendular movements of longitudinal muscle after tetrodotoxin (1 μmol l(-1)) treatment suggests these contractions were myogenic in origin. The present study highlights both similarities and differences in motility patterns between shorthorn sculpin and other vertebrates, as well as providing a platform to examine other aspects of gastrointestinal functions in fish, including the impact of environmental changes.

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.

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.

Postprandial changes in enteric electrical activity and gut blood flow in rainbow trout (Oncorhynchus mykiss) acclimated to different temperatures

Enteric electrical activity, cardiac output and gut blood flow were measured in rainbow trout (Oncorhynchus mykiss) acclimated to either 10 degrees C or 16 degrees C. Enteric electrical activity showed, in both the fasted and postprandial state, a distinct pattern with clusters of burst-like events interspersed by silent periods. The frequency of electrical events increased postprandially for both acclimation groups. Event frequency increased from 3.0+/-0.5 to 9.6+/-1.4 events min(-1) and from 5.9+/-0.9 to 11.8+/-2.0 events min(-1) in the 10 degrees C and 16 degrees C groups, respectively. Similarly, the number of events per cluster increased postprandially for both acclimation groups. Gut blood flow, cardiac output and heart rate increased after feeding. The gut blood flow significantly increased in both groups and peaked at 257+/-19% and 236+/-22% in the 10 degrees C and 16 degrees C groups, respectively. There was a strong correlation between the number of events and gut blood flow at both temperatures. Comparison between the two groups showed that fish acclimated to 16 degrees C may have an increased cost of sustaining the basal activity of the gut compared with the group acclimated to 10 degrees C. In conclusion, we have for the first time measured enteric electrical activity in vivo in a fish species and we have also demonstrated a strong correlation between gut blood flow and enteric electrical activity in fasted and postprandial fish.

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.

Development of enteric and vagal innervation of the zebrafish (Danio rerio) gut

The autonomic nervous system develops following migration and differentiation of precursor cells originating in the neural crest. Using immunohistochemistry on intact zebrafish embryos and larvae we followed the development of the intrinsic enteric and extrinsic vagal innervation of the gut. At 3 days postfertilization (dpf), enteric nerve cell bodies and fibers were seen mainly in the middle and distal intestine, while the innervation of the proximal intestine was scarcer. The number of fibers and cell bodies gradually increased, although a large intraindividual variation was seen in the timing (but not the order) of development. At 11-13 dpf most of the proximal intestine received a similar degree of innervation as the rest of the gut. The main intestinal branches of the vagus were similarly often already well developed at 3 dpf, entering the gut at the transition between the proximal and middle intestine and projecting posteriorly along the length of the gut. Subsequently, fibers branching off the vagus innervated all regions of the gut. The presence of several putative enteric neurotransmitters was suggested by using markers for neurokinin A (NKA), pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP), nitric oxide, serotonin (5-hydroxytryptamine, 5-HT), and calcitonin gene-related peptide (CGRP). The present results corroborate the belief that the enteric innervation is well developed before the onset of feeding (normally occurring around 5-6 dpf). Further, the more detailed picture of how development proceeds at stages previously not examined suggests a correlation between increasing innervation and more regular and elaborated motility patterns.

TTX-sensitive and TTX-insensitive control of spontaneous gut motility in the developing zebrafish (Danio rerio) larvae

Spontaneous regular gut motility in zebrafish begins around 4 days post fertilisation (d.p.f.) and is modulated by release of acetylcholine and nitric oxide. The role of intrinsic or extrinsic innervation for initiating and propagating the spontaneous contractions, however, is not well understood. By creating spatiotemporal maps, we could examine spontaneous motility patterns in zebrafish larvae in vivo at 4 and 7 d.p.f. in more detail. Tetrodotoxin (TTX) was added to elucidate the importance of nervous control. Anterograde and retrograde contraction waves originated in the same region,just posterior to the intestinal bulb. This area correlates well with the distribution of Hu (human neuronal protein C/D)-immunoreactive nerve cell bodies. Whereas numerous immunoreactive nerve cells were present in the mid and distal intestine at both 4 and 7 d.p.f., fewer cells were seen anterior to the origin of contractions. The overall frequency of contractions(1.16±0.15 cycles min–1, N=14 at 4 d.p.f.;1.05±0.09 cycles min–1, N=13 at 7 d.p.f.) and the interval between individual anterograde contraction waves (54.8±7.9 s at 4 d.p.f., N=14; 56.9±4.4 s, N=13 at 7 d.p.f.)did not differ between the two stages but the properties of the contractions were altered. The distance travelled by each wave increased from 591.0±43.8 μm at 4 d.p.f. (N=14) to 719.9±33.2 μm at 7 d.p.f. (N=13). By contrast, the velocity decreased from 4 d.p.f.(49.5±5.5 μm s–1, N=12) to 7 d.p.f.(27.8±3.6 μm s–1, N=13). At 4 d.p.f., TTX did not affect any of the parameters whereas at 7 d.p.f. anterograde frequency(control 1.07±0.12 cycles min–1, N=8; TTX 0.55±0.13 cycles min–1, N=8) and distance travelled (control 685.1±45.9 μm, N=8; TTX 318.7±88.7 μm, N=6) were decreased. In conclusion, enteric or extrinsic innervation does not seem to be necessary to initiate spontaneous contractions of the gut in zebrafish larvae. However, later in development,nerves have an increasingly important role as modulators of intestinal activity.)

Ontogeny of excitatory and inhibitory control of gastrointestinal motility in the African clawed frog,Xenopus laevis

The transparent body wall of Xenopus laevis larvae during the first developmental stages allows in vivo studies of gastrointestinal tract activity. The purpose of this study was to chart the ontogeny of gut motility in Xenopus larvae and to identify the most important control systems during the first developmental stages. Coordinated descending contraction waves first occurred in the gut at Nieuwkoop and Faber stage 43 [0.8 ± 0.1 contractions/min (cpm)] and increased to 4.9 ± 0.1 cpm at stage 47. The cholinergic receptor agonist carbachol (5–10 μM) increased contraction frequency already at stage 43, as did neurokinin A (NKA, 0.3–1 μM). The muscarinic antagonist atropine (100 μM) first affected contraction frequency at stage 45, which coincides with the onset of feeding. The tachykinin antagonist MEN-10,376 (6 μM) blocked NKA-induced contractions but not spontaneous motility. Both sodium nitroprusside [nitric oxide (NO) donor, 1–10 μM] and vasoactive intestinal peptide (VIP, 0.1–1 μM) inhibited contractions from the earliest stage onward. Blocking NO synthesis using NG-nitro-l-arginine methyl ester (100 μM) had no effect per se, but antagonized VIP evoked inhibition at stage 47. We conclude that gastrointestinal motility is well developed in the Xenopus laevis larvae before the onset of feeding. Functional muscarinic and tachykinin receptors are present already at the onset of motility, whereas a cholinergic tone develops around the onset of feeding. No endogenous tachykinin tone was found. Functional VIP receptors mediate inhibition at the onset of motility. NO seems to mediate the VIP effect at later stages.

The effects of endogenous and exogenous nitric oxide on gut motility in zebrafish Danio rerio embryos and larvae

Using motion analysis, the ontogeny of the nitrergic control system in the gut was studied in vivo in zebrafish Danio rerio embryos and larvae. For the first time we show the presence of a nitrergic tonus,modulating both anterograde and retrograde contraction waves in the intestine of developing zebrafish. At 4 d.p.f. (days post fertilisation), the nitric oxide synthase (NOS) inhibitor l-NAME (three boluses of 50–100 nl, 10–3 mol l–1) increased the anterograde contraction wave frequency by 0.50±0.10 cycles min–1. Subsequent application of the NO donor sodium nitroprusside (SNP; three boluses of 50–100 nl, 10–4mol l–1) reduced the frequency of propagating anterograde waves (–0.71±0.20 cycles min–1). This coincided with the first appearance of an excitatory cholinergic tonus, observed in an earlier study. One day later, at 5 d.p.f., in addition to the effect on anterograde contraction waves, application of l-NAME increased(0.39±0.15 cycles min–1) and following SNP application reduced (–1.61±0.36 cycles min–1) the retrograde contraction wave frequency. In contrast, at 3 d.p.f., when no spontaneous motility is observed, application of l-NAME did not induce contraction waves in either part of the gut, indicating the lack of a functional inhibitory tonus at this early stage. Gut neurons expressing NOS-like immunoreactivity were present in the distal and middle intestine as early as 2 d.p.f., and at 1 day later in the proximal intestine. In conclusion, the present study suggests that a nitrergic inhibitory tonus develops shortly before or at the time for onset of exogenous feeding.

Ontogeny of the gut motility control system in zebrafish Danio rerio embryos and larvae

Using digital motion analysis, the ontogeny of the cholinergic, tachykinin and pituitary adenylate cyclase-activating polypeptide (PACAP) control systems was studied in zebrafish Danio rerio larvae, in vivo. For the first time we show that the regular propagating anterograde waves that occur in the zebrafish larval gut before and around the onset [at 5–6 days post fertilization (d.p.f.)] of feeding are modulated by acetylcholine or atropine, PACAP and NKA (neurokinin A). At 3 d.p.f., when no spontaneous motility has developed, application of acetylcholine did not affect the gut. However, at 4 d.p.f., acetylcholine increased and atropine reduced the frequency of propagating anterograde waves. At 5 d.p.f., NKA increased and PACAP reduced the wave frequency. This suggests that both excitatory and inhibitory pathways develop at an early stage in the gut, independent of exogenous feeding. Immunohistochemistry established the presence of gut neurons expressing PACAP and NKA in the proximal part of the developing gut from the first stage investigated (2 d.p.f.) and before regular motility was observed. 1 d.p.f. (PACAP) or 2 d.p.f. (NKA) stages later the whole gut was innervated. This supports physiological results that gut motility is under neuronal control during the period when regular motility patterns develop.

Effects of digestive status on the reptilian gut

Reptiles, including the Burmese python, Python molurus bivittatus, that feed at infrequent intervals show a prominent increase in gastrointestinal mass, metabolism and brush border transport rates after feeding. Current knowledge and theories around these phenomena, as well as studies on the innervation of the reptilian gut, are summarised in this review. Little is known about the putative changes in the nervous and humoral control systems of the gut, and it is not known whether feeding affects innervation and motility of the stomach and intestine. Using immunohistochemistry, we have investigated possible up/down regulation of several neurotransmitters in specimens that had been fasted for a minimum of 3 weeks and specimens that had ingested a large meal 2 days before the experiments were conducted. There were no major changes in the innervation by nerves containing calcitonin gene-related peptide (CGRP), galanin, nitric oxide synthase (NOS), pituitary adenylate cyclase-activating polypeptide (PACAP), somatostatin (SOM), substance P/neurokinin A (SP/NKA), or vasoactive intestinal polypeptide (VIP)-like immunoreactivity. Nor did we find any differences in the effect of substance P (stomach and intestine), galanin (intestine), or bradykinin (intestine) on motility in strip preparations from the gut wall. A significant increase in dry weight of the intestine was obtained 48 h after feeding. We conclude that although there are considerable changes in gut thickness and absorptive properties after feeding, the smooth muscle and its control appear little affected.

The control of gut motility

Gut motility in non-mammalian vertebrates as in mammals is controlled by the presence of food, by autonomic nerves and by hormones. Feeding and the presence of food initiates contractions of the stomach wall and subsequently gastric emptying, peristalsis, migrating motor complexes and other patterns of motility follow. This overview will give examples of similarities and differences in control systems between species. Gastric receptive relaxation occurs in fish and is an enteric reflex. Cholecystokinin reduces the rate of gastric emptying in fish as in mammals. Inhibitory control of peristalsis is exerted, e.g. by VIP, PACAP, NO in fish and amphibians, while excitatory stimuli arise from nerves releasing tachykinins, acetylcholine or serotonin (5-HT). In crocodiles, we have found the presence of the same nerve types, although the effects on peristalsis have not been studied. Recent studies on signal transduction in the gut smooth muscle of fish and amphibians suggest that external Ca2+ is of great importance, but not the only source of Ca2+ recruitment in tachykinin-, acetylcholine- or serotonin-induced contractions of rainbow trout and Xenopus gastrointestinal smooth muscle. The effect of acetylcholine involves reduction of cAMP-levels in the smooth muscle cells. It is concluded that, in general, the control systems in non-mammalian vertebrates are amazingly similar between species and animal groups and in comparison with mammals.

Cyclooxygenase-derived products, rather than nitric oxide, are endothelium-derived relaxing factor(s) in the ventral aorta of carp (Cyprinus carpio)

In some fish blood vessels, the existence of a NO (nitric oxide) system has been reported. We examined the possibility that this NO system acts as an endothelium-derived relaxing factor (EDRF) in carp aorta using the carp aorta alone and in a combined carp-rat aorta donor-detector system. Use of the typical NO stimulating agent in mammal acetylcholine (ACh) only induced constriction of the carp aorta. This response was not modified by denudation or by NO synthesis inhibition with N-nitro-L-arginine methyl ester. Neither the indirect NO stimulating agents bradykinin and histamine nor the direct NO releasers sodium nitroprusside (SNP) and SIN-1 induced vasorelaxation. Both SNP and ACh elevated the cGMP concentration in rat aorta, but not in carp aorta. In the aorta combination set-up, where carp served as a NO donor and rat aorta served as a NO detector, no relaxation of the rat aorta was observed. The calcium ionophore A23187, a known EDRF producer in mammals, induced relaxation of carp aorta through an endothelium- and cyclooxygenase-dependent mechanism. These results indicate that carp aorta does not produce NO as an EDRF nor does it respond to exogenously supplied NO. The major EDRF in carp is apparently a product(s) of cyclooxygenase metabolism.

PACAP and nitric oxide inhibit contractions in the proximal intestine of the atlantic cod, Gadus morhua

The possible inhibitory roles of pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP) and nitric oxide in the control of intestinal motility were investigated in the Atlantic cod, Gadus morhua. Circular and longitudinal smooth muscle preparations developed spontaneous contractions that were inhibited by atropine (10(−)(5)mol l(−)(1)). PACAP 27 and PACAP 38 (10(−)(7)mol l(−)(1)) reduced the amplitude of the contractions but did not usually affect the resting tension. In the circular preparations, the mean active force developed (above resting level; +/− s.e.m.) was reduced from 0. 62+/−0.18 mN to 0.03+/−0.03 mN (N=10) by PACAP 27 and from 0.53+/−0. 20 mN to 0.31+/−0.13 mN (N=7) by PACAP 38, while neither cod nor mammalian VIP (10(−)(10)-10(−)(6)mol l(−)(1)) had any effect. In the longitudinal preparations, PACAP 27 reduced the force developed from 1.58+/−0.22 mN to 0.44+/−0.25 mN (N=8) and PACAP 38 reduced it from 1.61+/−0.47 mN to 0.75+/−0.28 mN (N=5). The nitric oxide donor sodium nitroprusside (NaNP) almost abolished the contractions in the circular preparations, reducing the mean force developed from 0. 47+/−0.05 mN to 0.02+/−0.06 mN (10(−)(6)mol l(−)(1); N=9) and 0+/−0. 07 mN (10(−)(5)mol l(−)(1); N=8). In the longitudinal preparations, NaNP reduced the force developed from 2.03+/−0.36 mN to 0.33+/−0.22 mN (10(−)(6)mol l(−)(1); N=8) and 0.19+/−0.30 mN (10(−)(5)mol l(−)(1); N=8). The L-arginine analogue N(G)-nitro-L-arginine methyl ester (L-NAME; 3×10(−)(4)mol l(−)(1)) enhanced the contractions in both circular and longitudinal preparations, increasing the mean force developed from 0.51+/−0.12 mN to 0.94+/−0.21 mN (N=8) and from 1.49+/−0.36 mN to 3.34+/−0.67 mN (N=7), respectively. However, preincubation with L-NAME before a second addition of PACAP 27 (10(−)(7)mol l(−)(1)) did not affect the response to PACAP, neither did preincubation with the guanylate cyclase inhibitor 6-anilinoquinoline-5,8-quinone (LY83583; 10(−)(5)mol l(−)(1)), while the inhibitory response to NaNP (3×10(−)(7)mol l(−)(1)) was abolished by LY83583. The PACAP analogue PACAP 6–27 (3×10(−)(7)mol l(−)(1)) had no effect on the response to either NaNP (3×10(−)(7)mol l(−)(1)) or PACAP 27 (10(−)(8)mol l(−)(1)) in the circular preparations. These findings indicate the presence of both a cholinergic and a nitrergic tonus in the smooth muscle preparations of the cod. Although PACAP and NaNP both inhibit contractions, there is no evidence of any interactions between the two substances. In addition, NaNP, but not PACAP, probably acts via stimulating the production of cyclic GMP. In conclusion, both PACAP and nitric oxide may act as inhibitory transmitters, using distinct signalling pathways, in the control of intestinal motility in the Atlantic cod.

Cholecystokinin affects gastric emptying and stomach motility in the rainbow trout Oncorhynchus mykiss

In this study, we describe new methods for recording gastric emptying and in vivo measurements of intragastric pressure in fish. Using these methods, we investigated the effects of the sulphated octapeptide of cholecystokinin (CCK8) on gastric emptying and on stomach motility in vivo and in vitro. Gastric emptying of 99Tcm-labelled food was measured in swimming fish by using a gamma camera, counting consecutive 2.5 min periods for 18–42 h. After 20 h, 55.3+/−4.0 % of the labelled food remained in the stomach of the control fish (mean s.e.m., N=9). Vascular infusion of CCK8 (25 pmol kg-1 h-1) delayed gastric emptying so that 70.4+/−4.8 % of the labelled food remained in the stomach after 20 h (N=8). Gastric pressure changes in vivo were measured using a balloon surgically fitted into the cardiac or pyloric part of the stomach. In the cardiac part, intra-arterial infusion of CCK8 at 0.1 nmol kg-1 h-1 resulted in a decrease in the frequency and amplitude of rhythmic contractions, while higher doses started/increased contractions. Atropine blocked much of the basal contractile activity, but did not influence the CCK8-induced inhibition of contractile activity. The pyloric part of the stomach was unaffected by intra-arterial infusion of CCK8 or atropine. In vitro perfusion of the stomach (with a balloon placed in the cardiac part to record motility) with CCK8 at high concentrations (10(−7)mol l-1 and above) augmented the spontaneous contractions, while lower concentrations had inconsistent effects. In addition, CCK8 (10(−7) to 10(−6)mol l-1) decreased the amplitude of spontaneous contractions in longitudinal strip preparations, usually in combination with an increase in the resting tension. The decrease in amplitude was not affected by the nitric oxide synthase inhibitor NG-nitro-l-arginine methyl ester hydrochloride (L-NAME; 10(−4)mol l-1). Depending on the concentration and experimental arrangement, CCK8 had either inhibitory or excitatory effects on the cardiac stomach, suggesting the possible presence of different types of CCK receptor. We conclude that the predominant effect of CCK8 in vivo may be a slowing down of gastric emptying, presumably coinciding with a release of bile into the duodenum.

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.

Nitric oxide synthase- and neuropeptide Y-containing subpopulations of sympathetic neurons in the coeliac ganglion of the Atlantic cod, Gadus morhua, revealed by immunohistochemistry and retrograde tracing from the stomach

In this study retrograde tracing was used to locate sympathetic ganglion cells innervating the stomach of a teleost fish, Gadus morhua. A subpopulation of small neurons in the coeliac ganglion was retrogradely labelled after Fast Blue injection in the stomach wall. Neurons projecting to the myenteric plexus and muscle layers contained tyrosine hydroxylase immunoreactivity, and neurons projecting to submucosal layers and blood vessels contained neuropeptide Y immunoreactivity in addition to being tyrosine hydroxylase immunoreactive. A population of nitric oxide synthase containing tyrosine hydroxylase immunoreactive neurons was also found in the coeliac ganglion. These neurons were not frequently labelled after injection in any layer of the stomach. The presence of entero-enteric pathways was also surveyed, but too few enteric neurons were labelled with Fast Blue after injection in the coeliac ganglion to indicate a presence of an entero-enteric reflex. We conclude that in teleost fish, as previously reported in a variety of mammals, a pattern of target specific chemical coding of sympathetic neurons exists, but that all reflex systems of mammalian vertebrates are perhaps not present in fish.