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

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.

Nervous control of photophores in luminescent fishes

Functional studies of the autonomic innervation in the photophores of luminescent fishes are scarce. The majority of studies have involved either the stimulation of isolated photophores or the modulatory effects of adrenaline-induced light emission. The fish skin is a highly complex organ that performs a wide variety of physiological processes and receives extensive nervous innervations. The latter includes autonomic nerve fibers of spinal sympathetic origin having a secretomotor function. More recent evidence indicates that neuropeptide-containing nerve fibers, such as those that express tachykinin and its NK1 receptor, neuropeptide Y, or nitric oxide, may also play an important role in the nervous control of photophores. There is no anatomical evidence that shows that nNOS positive (nitrergic) neurons form a population distinct from the secretomotor neurons with perikarya in the sympathetic ganglia. The distribution and function of the nitrergic nerves in the luminous cells, however, is less clear. It is likely that the chemical properties of the sympathetic postganglionic neurons in the ganglia of luminescent fishes are target-specific, such as observed in mammals.

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.

Innervation of lung and heart in the ray-finned fish, bichirs

Anatomical and functional studies of the autonomic innervation in the lung and the heart of the bichirs are lacking. The present review paper describes the presence of nerve fibers located in the muscle layers of the lung and its submucosa, the collection of unipolar neurons found in the submucosal and muscle layers of the glottis in a bichir species (Polypterus bichir bichir). Putative oxygen chemoreceptive, neuroepithelial cells (NECs) in the lung mucosa are also included. The latter share many immunohistochemical characteristics similar to those observed in the carotid body and neuroepithelial bodies of mammals. A packed collection of paraganglion cells is located within the trunk of the pulmonary vagus nerves. The paper also examines the occurrence of intracardiac neurons and nerve fibers in the heart of the above species. These studies show that various neurotransmitters may indicate different patterns of innervation in the lung and the heart of the bichirs. However, there is still much to be discovered about the lung and cardiovascular nervous control of these primitive fishes.

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.

Cloning and characterization of neuropeptide Y (NPY) and cocaine and amphetamine regulated transcript (CART) in Atlantic cod (Gadus morhua)

Neuropeptide Y (NPY) and cocaine and amphetamine regulated transcript (CART) are two neuropeptides involved in the regulation of feeding in both mammals and fish. NPY stimulates feeding whereas CART inhibits feeding. In this study, we have cloned the full-length cDNA and complete genomic DNA sequences for NPY and CART in Atlantic cod. Atlantic cod preproNPY share a 45-85% identity with preproNPY from other fish whereas preproCART shows a 70% identity to CART peptides from zebrafish and goldfish. RT-PCR revealed that NPY mRNA is expressed in brain, in particular the forebrain, and in peripheral tissues, including intestine and kidney. CART mRNA is expressed throughout the brain and in ovaries. In order to assess the role of these peptides in the regulation of feeding, we examined changes in mRNA expression in the forebrain before, during and after a meal. NPY and CART mRNA both undergo peri-prandial changes in expression, with NPY levels being elevated around meal time and CART showing a decline 2 h after a meal. Food deprivation for 7 days induced a decrease in CART mRNA expression in the brain but did not affect NPY mRNA expression. Overall, our results suggest that NPY and CART are conserved peptides that might be involved in the regulation of feeding and other physiological functions in Atlantic cod.

The Sympathetic Nervous System of Anamniotes

The sympathetic nervous system develops as an evolutionary trait with gnathostomes (jawed vertebrates), but not with agnathan fishes (i.e., hagfishes and lampreys). Organization of the sympathetic preganglionic neuronal columns is different in teleosts and anurans. In the teleosts so far examined, the majority of sympathetic preganglionic neurons (SPNs) are located in the dorsal part of the spinal central gray matter. In Tetraodontiformes, the cell column occupies only two rostral spinal segments, which are distinct in their cytoarchitecture and projections. On the other hand, the SPNs of anurans form two cell columns segregated mediolaterally. The lateral and medial columns are also distinct in their cytoarchitecture and projections. The neuroactive substances expressed in the SPNs both in teleosts and anurans are coded to the projections. In anurans, the SPNs containing gonadotrophin-releasing hormone and those containing calcitonin gene-related peptide are involved in the regulation of blood vessels and cutaneous glands, respectively. In the filefish, the SPNs containing galanin project specifically to non-adrenergic non-cholinergic postganglionic neurons in the cranial sympathetic ganglia. Therefore, both anuran and teleost systems have different morphological and chemical-coded patterns for functional variation, although the anuran sympathetic nervous system has more organizational similarity with that of amniotes.

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.

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.

Characterization of the effects of vasoactive substances on the bulbus arteriosus of the eel, Anguilla rostrata

The fish bulbus arteriosus (BA) smooths cardiac output by expanding during cardiac systole and rebounding during diastole, thereby providing constant perfusion of the gills downstream. Published data have demonstrated innervation of the teleost BA and shown that the tension and compliance of the BA responded to vasoactive agonists, such as epinephrine and acetylcholine, suggesting that the BA was more than a mere "windkessel." To examine vasoactivity in the BA more directly, we measured the responses of isolated tissue rings from the BA of the eel, Anguilla rostrata to a suite of putative vasoactive agonists, which had been shown to affect vascular smooth muscle in a variety of teleosts. The BA of the eel was insensitive to acetylcholine but constricted when endothelin (ET-1) was applied. Nitric oxide, sodium nitroprusside (SNP; NO donor), natriuretic peptides (NP), and prostaglandin E1 (but not the prostacyclin agonist carbaprostacyclin) produced significant dilation in the BA. Since both ET-1 and sarafotoxin S6c produced concentration-dependent constriction, it appears that endothelin receptor B-type (ETB) receptors (and possibly ETA receptors) are present. The dilation produced by SNP was also concentration dependent, as were the dilations produced by porcine C-type natriuretic peptide, eel atrial natriuretic peptide (NP receptor agonists), Sulprostone and Butaprost (PGE receptor agonists). Our data demonstrate that the BA of eel is responsive to a variety of vasoactive agonists, suggesting that the BA is under neurohumoral control. The role of agonist-induced changes in BA tension in fish cardiovascular physiology remains to be determined, as do the specific receptor types involved.

Neuropeptide Y effects on vasorelaxation and intestinal contraction in the Atlantic cod Gadus morhua

Neuropeptide Y (NPY) has prominent cardiovascular effects in mammals and sharks, but no such effect has previously been demonstrated in any teleost fish. In the Atlantic cod, we found that cod NPY (10(-10)-10(-6) M) relaxed celiac arteries precontracted with epinephrine, and weak contractions were elicited in intestinal ring preparations. A few NPY-immunoreactive nerve fibers were present along small gut arteries. The results suggest that cod NPY produces vasorelaxation both by a direct action on smooth muscle and by release of prostaglandins, but with no involvement of nitric oxide, leukotrienes, or endothelium-derived relaxing factors. An additional indirect effect involving another neurotransmitter may occur. Cod NPY (10(-7) M) and human NPY (10(-7) M) had identical effects on the vessels. Small differences only in the effects of porcine [Leu(31),Pro(34)]NPY, NPY-(13-36), and cod NPY suggest the presence of a Y(1) subfamily receptor, similar to the zebrafish Ya receptor. A physiological role for NPY in teleost vasculature is concluded, but surprisingly the effect, a vasodilation, is opposite to that in mammals and is mediated by prostaglandins.

Immunohistochemical localization of nNOS in the head kidney of larval and juvenile rainbow trout, Oncorhynchus mykiss

The aim of this investigation was to assess whether in teleosts, as in mammals, nitric oxide (NO) is involved in the regulation of cellular activity in the adrenal homolog. Larval and juvenile stages of the rainbow trout, Oncorhynchus mykiss, were used, in which the adrenal homolog consists of chromaffin adrenergic and interrenal steroidogenic cells localized mainly in the head kidney where there are also ganglion cells and nerve fibres that innervate the gland. In 12-month-old juveniles, the immunohistochemical reaction for neuronal nitric oxide synthase (nNOS), which catalyzes the synthesis of NO, revealed the presence of this enzyme in some nerve fibres and ganglion cells and only rarely in chromaffin cells. The latter are identified by the immunohistochemical reaction for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyltransferase (PNMT). In larvae at 27 days postfertilization, numerous cells dispersed in the head kidney are nNOS positive, whereas the TH and PNMT positive cells are very rare. At hatching (31 days postfertilization), the positivity for nNOS in the cells of the head kidney disappears and reappears at 60 days posthatching in some nerve cells and fibres. These results suggest an involvement of NO in the regulation of adrenal function as in mammals and the nature of nNOS positive cells present in the head kidney of larvae of 27 days is discussed.

Vasoactivity of the ventral aorta of the American eel (Anguilla rostrata), Atlantic hagfish ( Myxine glutinosa), and sea lamprey (Petromyzon marinus)

To determine if vascular smooth muscle from teleost and agnathan fishes expresses receptors for signaling agents that are important in vascular tension in other vertebrates, we exposed rings of aortic vascular smooth muscle from the eel (Anguilla rostrata), the hagfish (Myxine glutinosa), and the lamprey (Petromyzon marinus) to a suite of putative agonists, including: acetylcholine, endothelin, nitric oxide, natriuretic peptides, and prostanoids. Acetylcholine constricted aortic rings from the eel, but had no effect on the rings from lamprey. On the other hand, endothelin constricted rings from all three species. Use of receptor-specific ET agonists demonstrated that only ET(A) receptors are expressed in the eel and lamprey aorta. The nitric oxide donor sodium nitroprusside or nitric oxide itself dilated rings from the eel, but both agonists constricted rings from the hagfish and NO produced a biphasic response (constriction followed by dilation) in the lamprey. Two natriuretic peptides, eel ANP and porcine CNP, produced marginally significant dilation in the eel aorta, human ANP dilated the hagfish rings, and pCNP and eANP dilated the lamprey rings. The prostanoids PGE(1) and PGE(2) both dilated the eel aortic rings, and PGE(1) and carbaprostacyclin (stable PGI(2) agonist) dilated the hagfish and lamprey rings. Our results suggest that receptors for a variety of vasoactive signaling agents are expressed in the aortic smooth muscle of the earliest vertebrates (lamprey and hagfish), as well as the more advanced teleosts (eel).

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.

Contractile properties of the elasmobranch rectal gland

The importance of the rectal gland in elasmobranch osmoregulation is well established. The rate of secretion by the gland is under the control of a variety of secretagogues and inhibitors. Early morphological work suggested that a band of smooth muscle cells surrounds the periphery of the shark rectal gland between the secretory tubules and the connective tissue capsule. To confirm the presence of the muscle ring, we examined histological sections from two species of shark, Squalus acanthias and Carcharodon carcharius, and from the stingray Dasyatis sabina and stained sections from S. acanthias with the actin-specific ligand phalloidin. In all three species, a distinct band of what appeared to be smooth muscle cells was evident, and the putative muscle ring in S. acanthias stained specifically with phalloidin. Moreover, isolated rings of rectal gland tissue from S. acanthias constricted when acetylcholine or endothelin was applied and responded to nitric oxide with an initial dilation, followed by a more substantial constriction. Subsequent addition of porcine C-type natriuretic peptide dilated the rings, but two prostanoids (carbaprostacyclin and prostaglandin E(1)) did not change ring tension significantly. The rings did not respond to the endothelin-B-specific agonist sarafotoxoin S6c, suggesting that the response to endothelin was mediated via endothelin-A-type receptors. Our data confirm the presence of a smooth muscle ring in the periphery of the elasmobranch rectal gland and demonstrate that the gland responds to a suite of smooth muscle agonists, suggesting that changes in the dimensions of the whole rectal gland may play a role in its secretory function.

Pathway specific expression of neuropeptides and autonomic control of the vasculature

In this article, we review the immunohistochemical evidence for the pathway-specific expression of co-existing neuropeptides in autonomic vasomotor neurons, and examine the functional significance of these expression patterns for the autonomic regulation of the vasculature. Most final motor neurons in autonomic vasomotor pathways contain neuropeptides in addition to non-peptide co-transmitters such as catecholamines, acetylcholine and nitric oxide. Neuropeptides also occur in preganglionic vasomotor neurons. The precise combinations of neuropeptides expressed by neurons in vasomotor pathways vary with species, vascular bed, and the level within the vascular bed. This applies to both vasoconstrictor and vasodilator pathways. There is a similar degree of variation in the expression of neuropeptide receptors in the vasculature. Consequently, the contributions of different peptides to autonomic vasomotor control are closely matched to the functional requirements of specific vascular beds. This arrangement allows for a high degree of precision in vascular control in normal conditions and has the potential for considerable plasticity under pathophysiological conditions.

Distinct localization and target specificity of galanin-immunoreactive sympathetic preganglionic neurons of a teleost, the filefish Stephanolepis cirrhifer

Immunoreactivity for galanin was examined in the sympathetic preganglionic neurons in the spinal cord, adrenal glands, sympathetic ganglia, and some sensory ganglia of the filefish Stephanolepis cirrhifer. Galanin-immunoreactive neurons were found only in the rostral part, but not in the caudal part of the central autonomic nucleus (a column of sympathetic preganglionic neurons of teleosts). Many galanin-immunoreactive nerve terminals were found in contact with neurons in the celiac ganglia and the cranial sympathetic ganglia on both sides of the body. Most neurons encircled by galanin-immunoreactive nerve fibers were negative for tyrosine hydroxylase. Galanin-immunoreactive nerve fibers were very sparse in the spinal sympathetic paravertebral ganglia. No galanin-immunoreactive nerve fibers were found in the adrenal glands. No sensory neurons of the trigeminal, vagal, or spinal dorsal root ganglia were positive for galanin-immunoreactivity. These results suggest that galanin-immunoreactive sympathetic preganglionic neurons have distinct segmental localization and might project specifically to a population of non-adrenergic sympathetic postganglionic neurons in the celiac and cranial sympathetic ganglia.