Partial cloning of constitutive and inducible nitric oxide synthases and detailed neuronal expression of NOS mRNA in the cerebellum and optic tectum of adult Atlantic salmon (Salmo salar)

Nitric Oxide Function and Nitric Oxide Synthase Evolution in Aquatic Chordates

Nitric oxide (NO) is a key signaling molecule in almost all organisms and is active in a variety of physiological and pathological processes. Our understanding of the peculiarities and functions of this simple gas has increased considerably by extending studies to non-mammal vertebrates and invertebrates. In this review, we report the nitric oxide synthase (Nos) genes so far characterized in chordates and provide an extensive, detailed, and comparative analysis of the function of NO in the aquatic chordates tunicates, cephalochordates, teleost fishes, and amphibians. This comprehensive set of data adds new elements to our understanding of Nos evolution, from the single gene commonly found in invertebrates to the three genes present in vertebrates.

Expression Pattern of nos1 in the Developing Nervous System of Ray-Finned Fish

Fish have colonized nearly all aquatic niches, making them an invaluable resource to understand vertebrate adaptation and gene family evolution, including the evolution of complex neural networks and modulatory neurotransmitter pathways. Among ancient regulatory molecules, the gaseous messenger nitric oxide (NO) is involved in a wide range of biological processes. Because of its short half-life, the modulatory capability of NO is strictly related to the local activity of nitric oxide synthases (Nos), enzymes that synthesize NO from L-arginine, making the localization of Nos mRNAs a reliable indirect proxy for the location of NO action domains, targets, and effectors. Within the diversified actinopterygian nos paralogs, nos1 (alias nnos) is ubiquitously present as a single copy gene across the gnathostome lineage, making it an ideal candidate for comparative studies. To investigate variations in the NO system across ray-finned fish phylogeny, we compared nos1 expression patterns during the development of two well-established experimental teleosts (zebrafish and medaka) with an early branching holostean (spotted gar), an important evolutionary bridge between teleosts and tetrapods. Data reported here highlight both conserved expression domains and species-specific nos1 territories, confirming the ancestry of this signaling system and expanding the number of biological processes implicated in NO activities.

Nitric Oxide Production and Regulation in the Teleost Cardiovascular System

Nitric Oxide (NO) is a free radical with numerous critical signaling roles in vertebrate physiology. Similar to mammals, in the teleost system the generation of sufficient amounts of NO is critical for the physiological function of the cardiovascular system. At the same time, NO amounts are strictly controlled and kept within basal levels to protect cells from NO toxicity. Changes in oxygen tension highly influence NO bioavailability and can modulate the mechanisms involved in maintaining the NO balance. While NO production and signaling appears to have general similarities with mammalian systems, the wide range of environmental adaptations made by fish, particularly with regards to differing oxygen availabilities in aquatic habitats, creates a foundation for a variety of in vivo models characterized by different implications of NO production and signaling. In this review, we present the biology of NO in the teleost cardiovascular system and summarize the mechanisms of NO production and signaling with a special emphasis on the role of globin proteins in NO metabolism.

Pattern of nitrergic cells and fibers organization in the central nervous system of the Australian lungfish, Neoceratodus forsteri (Sarcopterygii: Dipnoi)

The Australian lungfish Neoceratodus forsteri is the only extant species of the order Ceratodontiformes, which retained most of the primitive features of ancient lobe finned-fishes. Lungfishes are the closest living relatives of land vertebrates and their study is important for deducing the neural traits that were conserved, modified, or lost with the transition from fishes to land vertebrates. We have investigated the nitrergic system with neural nitric oxide synthase (NOS) immunohistochemistry and NADPH-diaphorase (NADPH-d) histochemistry, which yielded almost identical results except for the primary olfactory projections and the terminal and preoptic nerve fibers labeled only for NADPH-d. Combined immunohistochemistry was used for simultaneous detection of NOS with catecholaminergic, cholinergic, and serotonergic structures, aiming to establish accurately the localization of the nitrergic elements and to assess possible interactions between these neurotransmitter systems. The results demonstrated abundant nitrergic cells in the basal ganglia, amygdaloid complex, preoptic area, basal hypothalamus, mesencephalic tectum and tegmentum, laterodorsal tegmental nucleus, reticular formation, spinal cord, and retina. In addition, low numbers of nitrergic cells were observed in the olfactory bulb, all pallial divisions, lateral septum, suprachiasmatic nucleus, prethalamic and thalamic areas, posterior tubercle, pretectum, torus semicircularis, cerebellar nucleus, interpeduncular nucleus, the medial octavolateral nucleus, nucleus of the solitary tract, and the dorsal column nucleus. Colocalization of NOS and tyrosine hydroxylase was observed in numerous cells of the ventral tegmental area/substantia nigra complex. Comparison with other vertebrates, using a neuromeric analysis, reveals that the nitrergic system of Neoceratodus shares many neuroanatomical features with tetrapods and particularly with amphibians.

Nitric Oxide and the Neuroendocrine Control of the Osmotic Stress Response in Teleosts

The involvement of nitric oxide (NO) in the modulation of teleost osmoresponsive circuits is suggested by the facts that NO synthase enzymes are expressed in the neurosecretory systems and may be regulated by osmotic stimuli. The present paper is an overview on the research suggesting a role for NO in the central modulation of hormone release in the hypothalamo-neurohypophysial and the caudal neurosecretory systems of teleosts during the osmotic stress response. Active NOS enzymes are constitutively expressed by the magnocellular and parvocellular hypophysiotropic neurons and the caudal neurosecretory neurons of teleosts. Moreover, their expression may be regulated in response to the osmotic challenge. Available data suggests that the regulatory role of NO appeared early during vertebrate phylogeny and the neuroendocrine modulation by NO is conservative. Nonetheless, NO seems to have opposite effects in fish compared to mammals. Indeed, NO exerts excitatory effects on the electrical activity of the caudal neurosecretory neurons, influencing the amount of peptides released from the urophysis, while it inhibits hormone release from the magnocellular neurons in mammals.

Characterization and gene expression analysis of pacu (Piaractus mesopotamicus) inducible nitric oxide synthase (iNOS) following Aeromonas dhakensis infection

Nitric oxide (NO) is an important effector molecule which is involved in a myriad of biological processes, including immune responses against pathogens such as parasites, virus and bacteria. During the inflammatory processes in vertebrates, NO is produced by the inducible nitric oxide synthase (iNOS) enzyme in practically all nucleated cells to suppress or kill intracellular pathogens. The aim of the present study was to characterize the full coding region of the iNOS gene of pacu (Piaractus mesopotamicus), an economically and ecologically important South American fish species, and to analyze mRNA expression levels following intraperitoneal infection with the pathogenic bacterium Aeromonas dhakensis by means of quantitative real time PCR (qPCR). The results showed that the pacu iNOS transcript is 3237 bp in length, encoding a putative protein composed of 1078 amino acid residues. The amino acid sequence showed similarities ranging from 69.03% to 94.34% with other teleost fish and 57.70% with the human iNOS, with all characteristic domains and cofactor binding sites of the enzyme detected. Phylogenetic analysis showed that the iNOS from the red-bellied piranha, another South American characiform, was the closest related sequence to the pacu iNOS. iNOS transcripts were constitutively detected in the liver, spleen and head kidney, and there was a significant upregulation in the liver and spleen at 12, 24 and 48 h after infection with A. dhakensis. No significant variations were observed in the head kidney during the periods analyzed. These results show that iNOS expression was induced by A. dhakensis infection and suggest that this enzyme may be involved in the response to this bacterium in pacu.

The expression signatures of neuronal nitric oxide synthase (NOS1) in turbot (Scophthalmus maximus L.) mucosal surfaces against bacterial challenge

The mucosal surfaces constitute the first immune barrier of host defense and also serve as the dynamic interfaces that simultaneously mediate a diverse array of critical physiological processes. It has been long hypothesized that observed difference of disease resistance among different fish strains and species are strongly correlated to the activities of the immune actors in mucosal surfaces. Particularly, neuronal NOS (nNOS or NOS1) is a constitutively expressed gene that catalyzes the oxidation of l-arginine and water to nitric oxide (NO), which is known as a potent host defence effector in immune system with antimicrobial activity. Moreover, NOS1 was detected to be expressed in fish mucosal surfaces, but its activities in mucosal immune responses were always overlooked. In this regard, we identified the NOS1 of turbot and characterized its expression patterns in mucosal tissues following Vibrio anguillarum and Streptococcus iniae challenge. The results showed that the NOS1 gene had a 4389 bp open reading frame (ORF) that encoded 1462 amino acids. Phylogenetic analysis showed the turbot NOS1 had the strongest relationship to Larimichthys crocea. And the syntenic analysis revealed the similar neighboring genes associated with turbot NOS1, compared with other teleost and mammals. In addition, NOS1 was widely expressed in all examined tissues with the highest expression level in brain, followed by intestine and gill. Finally, the NOS1 showed a general trend of up-regulation in mucosal tissues following both bacterial challenge, with the highest up-regulation in intestine. The significant quick induction of NOS1 in mucosal surfaces against infection indicated its key roles to prevent pathogen attachment and entry in mucosal immunity. More functional studies are needed to conduct in teleost to better understand the roles of NOS1 in maintaining the integrity of the mucosal barriers against infection.

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.

Nitric oxide inhibition of NaCl secretion in the opercular epithelium of seawater-acclimated killifish, Fundulus heteroclitus

Nitric oxide (NO) modulates epithelial ion transport pathways in mammals, but this remains largely unexamined in fish. We explored the involvement of NO in controlling NaCl secretion by the opercular epithelium of seawater killifish using an Ussing chamber approach. Pharmacological agents were used to explore the mechanism(s) triggering NO action. A modified Biotin-switch technique was used to investigate S-nitrosation of proteins. Stimulation of endogenous NO production via the nitric oxide synthase (NOS) substrate l-arginine (2.0 mmol l^-1), and addition of exogenous NO via the NO donor SNAP (10^-6 to 10^-4 mol l^-1), decreased the epithelial short-circuit current (I_sc). Inhibition of endogenous NO production by the NOS inhibitor l-NAME (10^-4 mol l^-1) increased I_sc and revealed a tonic control of ion transport by NO in unstimulated opercular epithelia. The NO scavenger PTIO (10^-5 mol l^-1) supressed the NO-mediated decrease in I_sc, and confirmed that the effect observed was elicited by release of NO. The effect of SNAP on I_sc was abolished by inhibitors of the soluble guanylyl cyclase (sGC), ODQ (10^-6 mol l^-1) and Methylene Blue (10^-4 mol l^-1), revealing NO signalling via the sGC/cGMP pathway. Incubation of opercular epithelium and gill tissues with SNAP (10^-4 mol l^-1) led to S-nitrosation of proteins, including Na⁺/K⁺-ATPase. Blocking of NOS with l-NAME (10^-6 mol l^-1) or scavenging of NO with PTIO during hypotonic shock suggested an involvement of NO in the hypotonic-mediated decrease in I_sc Yohimbine (10^-4 mol l^-1), an inhibitor of α₂-adrenoceptors, did not block NO effects, suggesting that NO is not involved in the α-adrenergic control of NaCl secretion.

Organization of the nitrergic neuronal system in the primitive bony fishes Polypterus senegalus and Erpetoichthys calabaricus (Actinopterygii: Cladistia)

Cladistians are a group of basal actinopterygian fishes that constitute a good model for studying primitive brain features, most likely present in the ancestral bony fishes. The analysis of the nitrergic neurons (with the enzyme nitric oxide synthase; NOS) has helped in understanding important aspects of brain organization in all vertebrates studied. We investigated the nitrergic system of two cladistian species by means of specific antibodies against NOS and NADPH-diaphorase (NADPH-d) histochemistry, which, with the exception of the primary olfactory and terminal nerve fibers, labeled only for NADPH-d, yielded identical results. Double immunohistochemistry was conducted for simultaneous detection of NOS with tyrosine hydroxylase, choline acetyltransferase, calbindin, calretinin, and serotonin, to establish accurately the localization of the nitrergic neurons and fibers and to assess possible interactions between these neuroactive substances. The pattern of distribution in both species showed only subtle differences in the density of labeled cells. Distinct groups of NOS-immunoreactive cells were observed in pallial and subpallial areas, paraventricular region, tuberal and retromammillary hypothalamic areas, posterior tubercle, prethalamic and thalamic areas, optic tectum, torus semicircularis, mesencephalic tegmentum, interpeduncular nucleus, superior and middle reticular nuclei, magnocellular vestibular nucleus, solitary tract nucleus, nucleus medianus magnocellularis, the spinal cord and amacrine cells in the retina. Large neurons in cranial nerve sensory ganglia were also labeled. The comparison of these results with those from other vertebrates, using a neuromeric analysis, reveals a conserved pattern of organization of the nitrergic system from this primitive fish group to amniotes, including mammals.

Expression of nitric oxide synthase (NOS) genes in channel catfish is highly regulated and time dependent after bacterial challenges

Nitric oxide is well known for its roles in immune responses. As such, its synthesizing enzymes have been extensively studied from various species including some teleost fish species. However, the NOS genes have not been characterized in channel catfish (Ictalurus punctatus). In this study, we identified and characterized three NOS genes including one NOS1 and two NOS2 genes in channel catfish. Comparing with the NOS genes from other fish species, the catfish NOS genes are highly conserved in their structural features. Phylogenetic and syntenic analyses allowed determination of NOS1 and NOS2 genes of channel catfish and their orthology relationships. Syntenic analysis, as well as the phylogenetic analysis, indicated that the two NOS2 genes of catfish were lineage-specific duplication. The NOS genes were broadly expressed in most tested tissues, with NOS1 being expressed at the highest levels in the brain, NOS2b1 highly expressed in the skin and gill, and NOS2b2 lowly expressed in most of the tested tissues. The most striking findings of this study was that the expression of the NOS genes are highly regulated after bacterial infection, with time-dependent expression patterns that parallel the migration of macrophages. After Edwardsiella ictaluri challenge, dramatically different responses among the three NOS genes were observed. NOS1 was only significantly in the skin early after infection, while NOS2b1 was rapidly upregulated in gill, but more up-regulated in trunk kidney with the progression of the disease, suggesting such differences in gene expression may be reflective of the migration of macrophages among various tissues of the infected fish. In contrast to NOS1 and NOS2b1, NOS2b2 was normally expressed at very low levels, but it is induced in the brain and liver while significantly down-regulated in most other tissues.

Molecular cloning and expression profiles of nitric oxide synthase (NOS) in mud crab Scylla paramamosain

The importance of the nitric oxide synthase (NOS) gene family is demonstrated by many studies in vertebrates and invertebrates in recent years. However, it keeps unknown of nitric oxide (NO) system and NOS gene family in mud crab Scylla paramamosain, an important cultured commercial crustacean in China and Pacific area. In this report, the cDNA of NOS containing full-length ORF was cloned from mud crab, S. paramamosain. It was of 4424 bp, including a 5'-terminal untranslated region (UTR) of 239 bp, a 3'-terminal UTR of 540 bp, which contained two ATTTA motifs, and an open reading frame (ORF) of 3645 bp encoding a polypeptide of 1214 amino acids. Structural analysis indicated that NOS contained a typical NO synthase domain at the N-terminal, next to a flavodoxin 1 domain, a flavin adenine dinucleotide (FAD) binding domain, respectively, and a conservative nicotinamide adenine dinucleotide (NAD) binding domain structure at the C-terminal. Quantitative real-time PCR analysis revealed S. paramamosain NOS (SpNOS) to be expressed in all tissues examined, with the highest expression in midintestine and the weakest level in heart and eyestalk. The expression profiles of SpNOS indicated that the NOS expression levels were significantly induced in midintestine, hepatopancrease and hemocytes after challenged with Vibrio Parahaemolyticus, the synthetic double-stranded RNA polyinosinic polycytidylic acid (poly I:C) and lipopolysaccharides (LPS). The NOS activity in hemocytes showed significant increase during at 24 h-48 h time period after immune challenges with V. Parahaemolyticus, poly I:C and LPS. Results here may suggest that the inducible NOS play an important role in mud crab's defense against pathogenic infection.

Anti-inflammatory effects of β-sitosterol-β-D-glucoside from Trachelospermum jasminoides (Apocynaceae) in lipopolysaccharide-stimulated RAW 264.7 murine macrophages

Trachelospermum jasminoides (Apocynaceae) has pharmacological effects that include anti-inflammatory, anti-bacterial and anti-viral activities, which have been observed from various studies. Of these pharmacological effects, the anti-inflammatory capacity of compounds from T. jasminoides is not yet known exactly. In this study, we investigated the compound that can be used for the suppression of lipopolysacchaide (LPS) stimulated inflammatory responses in macrophages among the five isolated compounds. β-sitosterol-β-D-glucoside (1) was found to reduce nitric oxide (NO) production from LPS-induced RAW 264.7 cells the most. In addition, compound 1 strongly inhibited the interleukin 6 (IL-6) activities of stimulated macrophages. Treatment of RAW 264.7 cells with compound 1 reduced secretion of inflammatory elements including tumour necrosis factor - alpha (TNF-α) and interleukin 1 beta (IL-1β). Thus, compound 1 may be a useful candidate for the development of new drugs to treat endotoxemia and inflammation accompanied by the overproduction of NO.

The unique mitochondrial form and function of Antarctic channichthyid icefishes

Antarctic icefishes of the family Channichthyidae are the only vertebrate animals that as adults do not express the circulating oxygen-binding protein hemoglobin (Hb). Six of the 16 family members also lack the intracellular oxygen-binding protein myoglobin (Mb) in the ventricle of their hearts and all lack Mb in oxidative skeletal muscle. The loss of Hb has led to substantial remodeling in the cardiovascular system of icefishes to facilitate adequate oxygenation of tissues. One of the more curious adaptations to the loss of Hb and Mb is an increase in mitochondrial density in cardiac myocytes and oxidative skeletal muscle fibers. The proliferation of mitochondria in the aerobic musculature of icefishes does not arise through a canonical pathway of mitochondrial biogenesis. Rather, the biosynthesis of mitochondrial phospholipids is up-regulated independently of the synthesis of proteins and mitochondrial DNA, and newly-synthesized phospholipids are targeted primarily to the outer-mitochondrial membrane. Consequently, icefish mitochondria have a higher lipid-to-protein ratio compared to those from red-blooded species. Elevated levels of nitric oxide in the blood plasma of icefishes, compared to red-blooded notothenioids, may mediate alterations in mitochondrial density and architecture. Modifications in mitochondrial structure minimally impact state III respiration rates but may significantly enhance intracellular diffusion of oxygen. The rate of oxygen diffusion is greater within the hydrocarbon core of membrane lipids compared to the aqueous cytosol and impeded only by proteins within the lipid bilayer. Thus, the proliferation of icefish's mitochondrial membranes provides an optimal conduit for the intracellular diffusion of oxygen and compensates for the loss of Hb and Mb. Currently little is known about how mitochondrial phospholipid synthesis is regulated and integrated into mitochondrial biogenesis. The unique architecture of the oxidative muscle cells of icefishes highlights the need for further studies in this area.

Relationship among circulating hemoglobin, nitric oxide synthase activities and angiogenic poise in red- and white-blooded Antarctic notothenioid fishes

Nitric oxide (NO)-mediated angiogenesis may play a role in establishing dense retinal vasculatures of Antarctic hemoglobinless icefishes (suborder: Notothenioidei). We hypothesized that loss of hemoglobin (Hb) leads to elevation in [NO] due to decreased degradation of the compound when the NO-scavenger Hb is absent, thereby inducing vascular growth. We found that total mass of NO metabolites, nitrite plus nitrate (NO(x)), in plasma is greater in icefishes than in red-blooded notothenioids [e.g. C. aceratus (Hb-), 22.7+/-2.9 microM; N. coriiceps (Hb+), 14.7+/-1.7 microM], suggesting a higher NO load in hemoglobinless animals. High NO levels do not appear to be a result of greater NO synthesis; we consistently measured lower activities of the enzyme catalyzing NO production, nitric oxide synthase, in tissues of icefishes than in Hb-expressing notothenioids [e.g. 96+/-10 and 216+/-39 pmol(min g wet wt)(-1) in brain tissue of C. aceratus (Hb-) and G. gibberifrons (Hb+), respectively]. Levels of mRNA for hypoxia-induced (HIF-1alpha and PHD2) and angiogenic genes (VEGF) were similar in red- and white-blooded species, indicating that vascular maintenance in adult animals does not require differences in angiogenic tone. This does not preclude a cause-and-effect relationship between absence of Hb and NO-mediated angiogenesis during earlier ontogenetic stages of icefishes.

PACAP stimulation of maturational gonadotropin secretion in goldfish involves extracellular signal-regulated kinase, but not nitric oxide or guanylate cyclase, signaling

In goldfish, nitric oxide synthase (NOS) immunoreactivity is present in gonadotropes and extracellular signal-regulated protein kinase (ERK) mediates GnRH stimulation of gonadotropin release and synthesis. In this study, we tested the possible involvement of nitric oxide (NO) and ERK in mediating PACAP-stimulated maturational gonadotropin (GTH-II) release from primary cultures of dispersed goldfish pituitary cells. In static incubation experiments, PACAP-induced GTH-II release was unaffected by two inhibitors of NOS synthase, AGH and 1400W; whereas addition of a NO donor, SNAP, elevated GTH-II secretion. In perifusion experiments, neither NOS inhibitors (AGH, 1400W and 7-Ni) nor NO scavengers (PTIO and rutin hydrate) attenuated the GTH-II response to pulse applications of PACAP. In addition, the GTH-II responses to PACAP and the NO donor SNP were additive while PTIO blocked SNP action. Although dibutyryl cGMP increased GTH-II secretion in static incubation, inhibition of guanylate cyclase (GC), a known down-stream target for NO signaling, did not reduce the GTH-II response to pulse application of PACAP. On the other hand, GTH-II responses to PACAP in perifusion were attenuated in the presence of two inhibitors of ERK kinase (MEK), U 0126 and PD 98059. These results suggest that although increased availability of NO and cGMP can lead to increased GTH-II secretion, MEK/ERK signaling, rather than NOS/NO/GC activation, mediates PACAP action on GTH-II release in goldfish.

Effect of hyperosmotic stress on the gene expression and activity of neuronal nitric oxide synthase (nNOS) in the preoptic-hypothalamic neurosecretory system of the euryhaline fish Oreochromis mossambicus

We examined the effects of hyperosmotic stress on the gene expression and activity of neuronal nitric oxide synthase (nNOS) in the preoptic/hypothalamic neurosecretory system of the euryhaline tilapia Oreochromis mossambicus (Mozambique tilapia) by means of semiquantitative RT-PCR and NADPHd histochemistry. Expression of nos1 was rapidly and transiently up-regulated in the preoptic region and hypothalamus in response to a salinity change (70% seawater, SW). Expression levels increased 4 h after the salinity change and then returned to basal levels within 8 h of the hyperosmotic challenge. NADPHd histochemistry revealed that positive magnocellular and gigantocellular preoptic neurons increased in number 4 h after the salinity change, while the number of parvocellular preoptic neurons reactive for NADPHd showed no significant change. These results indicate that the nNOS gene expression and NOS activity are stimulated in the preoptic/ hypothalamic neurosecretory system in response to hyperosmotic stress and suggest that NO influences neuronal responses to short-term osmotic stimulation in euryhaline fish.

Purification and localization of nitric oxide synthases from hybrid tilapia (Nile tilapia x Mozambique tilapia)

The aims of this study were to purify and localize the nitric oxide synthases (NOSs) from hybrid tilapia (Nile tilapia Oreochromis niloticus x Mozambique tilapia O. mossambicus). The purification procedures involved affinity chromatography with a 2', 5'-ADP-agarose 4B column and ion exchange with a diethylaminoethanol Bio-Gel A column. The results from gel filtration assays showed that the molecular weights of neuronal NOS (nNOS) and inducible NOS (iNOS) were 178 and 120 kDa, respectively. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis results showed that there were three bands with molecular weights of 89, 47, and 29 kDa from the purified nNOS. However, only one band, with a molecular weight of 120 kDa, appeared on the gel from the purified iNOS. Hybrid tilapia nNOS was a dimer structure, while iNOS appeared to be a monomer structure. Moreover, our results revealed that the activities of nNOS and iNOS were significantly higher after the addition of Ca+2 or Mg+2 ions individually. However, when L-arginine and NADPH were present, the addition of 1 mM of either ion did not further increase the activity. The chemical L-N(G)-methyl-L-arginine could inhibit the activities of the purified NOSs with or without L-arginine. Western blot analyses showed only an 89-kDa immunoreactive band from the extracts of cerebrum; however, we did not find the specific bands in other tissues, such as gill, intestine, liver, spleen, and anterior kidney. We found another 120-kDa immunoreactive protein band with the rabbit antirat iNOS serum against iNOS from the extracts of anterior kidney and spleen. The results of immunohistochemistry with the rabbit antihuman nNOS serum indicated that the nNOS existed in the cerebellum, olfactory bulb, diencephalons, and nerve cell bodies and neuronal fibers of the spinal cord. Interestingly, only macrophages from anterior kidney and spleen showed positive reactions with the rabbit antirat iNOS serum. In the same way, the endothelial NOS (eNOS) located in the heart and epithelial cells of the blood vessels reacted positively with the rabbit antibovine eNOS serum.

Distribution of NADPH-diaphorase and nitric oxide synthase reactivity in the central nervous system of the goldfish (Carassius auratus)

The nitrergic system has been inferred from cells positive to nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry and/or to the neuronal isoform of nitric oxide synthase (nNOS) immunohistochemistry in different species of vertebrates. The aim of the present work was to systematically study the distribution of cell producing nitric oxide in the goldfish (Carassius auratus) brain. To reach this goal, we firstly studied co-localization for NADPHd and nNOS techniques and demonstrated an extensive double labeling. Then, we studied the distribution through the brain by the two separate methods and found labeled cells widely distributed in brain and spinal cord. In the telencephalon, such cells were in both dorsal and ventral areas. In the diencephalon, the cells were found in some nuclei of the preoptic area and hypothalamus, habenula, pretectum, and dorsal and ventral thalamic regions. In the midbrain, cells were observed in the optic tectum, torus longitudinalis, and tegmental nuclei. In the rhombencephalon, cells were found in the cerebellum, the reticular formation, the locus coeruleus, the raphe nuclei, and the nuclei of the cranial nerves. Labeled cells were also observed in the gray area of the spinal cord. Cognizing that a direct comparison of the present results with those reported in other vertebrates is not clear-cut because of homologies; we conclude that the nitrergic system is roughly similar from fish to mammals.

Cardiac expression and distribution of nitric oxide synthases in the ventricle of the cold-adapted Antarctic teleosts, the hemoglobinless Chionodraco hamatus and the red-blooded Trematomus bernacchii

The presence of nitric oxide synthase (NOS) was investigated in the ventricle of two Antarctic teleosts, the hemoglobinless icefish Chionodraco hamatus and its red-blooded counterpart, Trematomus bernacchii. Under unstimulated conditions, in both teleosts, NADPH-diaphorase localised NOS activity in the endocardial-endothelial cells (EEc) and in the myocardiocytes. Application of anti-mammalian endothelial and inducible NOS (eNOS and iNOS, respectively) primary antibodies for immunofluorescence revealed a comparable tissue-specific basal expression of the two isoforms in the two species. eNOS strongly localised at the level of the EEc and, in T. bernacchii, of the vascular endothelium (VE). The enzyme is also localised, albeit to less extent, within the myocardiocytes, and in the epicardium. In contrast, iNOS immunostaining only labels the cytoplasm of the ventricular myocytes. Western blotting analysis identified two peptides with molecular masses of about 135 and 130kDa, similar to those of the mammalian eNOS and iNOS. To verify whether this NOS system is susceptible to septic stimulation, C. hamatus and T. bernacchii were exposed to bacterial lipopolysaccharides (LPS). The treatment did not modify the distribution pattern of the two isoenzymes while it increased the amount of NADPH-diaphorase-dependent reaction product and the expression of both eNOS and iNOS. These results indicate a high phylogenetic conservation of the intracardiac NOS system, emphasizing its importance in the control of the vertebrate heart and its relevance as a general mechanism of defense against pathogens.

Neuronal nitric oxide synthase in the gill of the killifish, Fundulus heteroclitus

Neuronal NOS (nNOS) is a constitutively expressed enzyme that catalyzes the oxidation of L-arginine and water to L-citrulline and the gas nitric oxide (NO). Nitric oxide is involved in regulation of a variety of processes, including: vascular tone, neurotransmission, and ion balance in mammals and fishes. In this study, we have cloned and characterized a putative NOS homologue from the brain of the euryhaline killifish, Fundulus heteroclitus. Killifish NOS has 75% amino acid identity to human nNOS, and phylogenetic analysis groups the killifish sequence with the mammalian nNOS, suggesting that it is a mammalian orthologue. Relative quantitative reverse transcriptase-PCR demonstrated that killifish nNOS mRNA is highly expressed in the brain and gill followed by the stomach, kidney, opercular epithelium, intestine and heart. Immunohistochemistry localized nNOS to nerve fibers and epithelial cells adjacent to mitochondrion-rich cells (ion transporting cell) in the gill, suggesting that nNOS production of NO may contribute to regulation of vascular tone and/or MRC function in the teleost gill.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

Partial cloning of neuronal nitric oxide synthase (nNOS) cDNA and regional distribution of nNOS mRNA in the central nervous system of the Nile tilapia Oreochromis niloticus

A constitutive NOS complementary DNA (cDNA) was partially cloned by RT-PCR from the brain of a teleost, the Nile tilapia (Oreochromis niloticus), using degenerate primers against conserved regions of NOS. The predicted 206-long amino acid sequence showed a high degree of identity with other vertebrate neuronal NOS (nNOS) protein sequences. In addition, phylogenetic analysis revealed that Nile tilapia NOS clustered with other known nNOS. Using the coupled reaction of semi-quantitative RT-PCR and Southern blotting, the basal tissue expression pattern of the cloned nNOS gene was investigated in discrete areas of the central nervous system (CNS) and in the heart and skeletal muscle tissue. As revealed, expression of nNOS transcripts was detected in all the CNS regions examined, whereas nNOS gene was not expressed in the heart and skeletal muscle. The distribution pattern of nNOS gene expression showed the highest expression levels in the forebrain followed by the optic tectum, the brainstem and the spinal cord, whereas scarce expression was detected in the cerebellum. Cellular expression of nNOS mRNA was analyzed in the CNS by means of in situ hybridization. According to the RT-PCR results, most nNOS mRNA expressing neurons are localized in the telencephalon and diencephalon, whereas in the mesencephalic optic tectum, the brainstem and the spinal cord, nNOS mRNA expressing neurons are relatively more scattered. A very low hybridization signal was detected in the cerebellar cortex. These results suggest that NO is involved in numerous brain functions in teleosts.

NO as a mediator during the early development of the cardiovascular system in the zebrafish

As a general pattern innervation of the cardiovascular system appears late during development in vertebrate embryos, and cardiovascular control may be achieved by hormonal activity in early stages. However, very little is known about the onset of NO-responsiveness during development, which in adult vertebrates is known to play a key function in many physiological processes such as control of vascular tone, neurotransmission, macrophage activity, and angiogenesis. Analysis of the effect of NO on the cardiovascular system in zebrafish (Danio rerio) embryos and larvae revealed almost no effect on cardiac activity during chronic exposure to NO-producing chemicals, whereas vascular reactivity was observed in veins and arteries of the zebrafish in early developmental stages (5-6 days post fertilization). Chronic exposure also modified the development of the vascular system. The presence of an NO donor (sodium nitroprusside) did not change the patterning of the vascular bed, but it induced an earlier appearance of some blood vessels in the trunk region of the zebrafish larvae. The data reveal that NO plays an important role in the development of the cardiovascular system and in the ontogeny of the cardiovascular control system in fish.

Nitric oxide in control of luminescence from hatchetfish(Argyropelecus hemigymnus) photophores

Nitric oxide synthase-like immunoreactivity (NOS-LI IR) was detected by immunohistochemistry in ventral light organs of the mesopelagic fish, Argyropelecus hemigymnus. Strong NOS-LI IR was present in nerve fibres and in other cells central for production or modulation of light:immunoreactive fibres surrounded the photophores, and were also present in the filter area. Filter cells, particularly in the outer layers, showed strong IR throughout the cytoplasm. Pharmacological studies suggested that nitric oxide(NO) modulates adrenaline-stimulated light emission, and that the modulation is correlated to the ability of the light organ to respond to adrenaline. Adrenaline is known to produce two different types of light response in isolated photophores from Argyropelecus: a slow, long-lasting, high intensity response, or a fast and weak response of short duration. Incubation of photophores in the NO donors sodium nitroprusside or S-nitroso-N-acetylpenicillamine prior to adrenaline stimulation reduced the intensity of the strong and long-lasting type of response, but had little or even a potentiating effect on the weakly responding photophores. Hydroxylamine, which is converted to NO if catalase activity is present in the tissue, reduced the duration and the intensity of the adrenaline response in all tested organs. The NOS-inhibitor l-thiocitrulline potentiated the adrenaline response in the weakly responding organs; the weaker the adrenaline effect, the stronger the potentiation caused by l-thiocitrulline. The strongly responding organs were instead inhibited by l-thiocitrulline. The results suggest that NO has an important role in the control of light emission from Argyropelecus hemigymnus photophores. The cGMP analogue dibutyryl cGMP, the guanylate cyclase inhibitor ODQ and the phosphodiesterase inhibitor pentoxiphylline had no effect, indicating that the NO effect does not involve cGMP.

Nitric oxide synthase in the gill of Atlantic salmon: colocalization with and inhibition of Na+,K+-ATPase

We investigated the relationship between nitric oxide (NO) and Na+,K+-ATPase (NKA) in the gill of anadromous Atlantic salmon. Cells containing NO-producing enzymes were revealed by means of nitric oxide synthase (NOS) immunocytochemistry and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry, which can be used as an indicator of NOS activity, i.e. NO production. Antibodies against the two constitutive NOS isoforms, neuronal and endothelial NOS, both produced immunoreactivity restricted to large cells at the base and along the secondary lamellae. NADPHd-positive cells showed a corresponding distribution. Antibodies against the inducible NOS isoform only labeled small cells located deep in the filament. Using in situ hybridization and NKA immunoreactivity, cells expressing Na+,K+-ATPaseα-subunit mRNA were found to have a similar distribution to the NOS cells. Double labeling for NOS immunoreactivity and NKA α-subunit mRNA revealed cellular colocalization of NKA α-subunit mRNA and nNOS protein in putative chloride cells at the base of the lamellae and interlamellar space. Along the lamellae, some NOS- or NKA-immunoreactive cells possessed a relatively lower expression of NKA α-subunit mRNA in smolts. A clear increase in NADPHd staining in the gill was demonstrated from parr to smolt. The regulatory role of NO on gill NKA activity was studied in vitrousing sodium nitroprusside (SNP; 1 mmol l-1) and PAPA-NONOate(NOC-15; 0.5 mmol l-1) as NO donors. Both SNP and NOC-15 inhibited gill NKA activity by 30% when compared to controls. The study shows that NO systems are abundant in the gill of Atlantic salmon, that NO may be produced preferentially by a constitutive NOS isoform, and suggests that NO influence on gill functions is mediated via intracellular, possibly both auto/paracrine,inhibition of Na+,K+-ATPase activity in chloride cells. Furthermore, the increase in NADPHd in the gill during smoltification suggests a regulatory role of NO in the attenuation of the smoltification-related increase in Na+,K+-ATPase activity prior to entering seawater.

Nitric Oxide (NO) in normal and hypoxic vascular regulation of the spiny dogfish,Squalus acanthias

Nitric oxide (NO) is a potent vasodilator in terrestrial vertebrates, but whether vascular endothelial-derived NO plays a role in vascular regulation in fish remains controversial. To explore this issue, a study was made of spiny dogfish sharks (Squalus acanthias) in normoxia and acute hypoxia (60 min exposure to seawater equilibrated with 3% oxygen) with various agents known to alter NO metabolism or availability. In normoxia, nitroprusside (a NO donor) reduced blood pressure by 20%, establishing that vascular smooth muscle responds to NO. L-arginine, the substrate for NO synthase, had no hemodynamic effect. Acetylcholine, which stimulates endothelial NO and prostaglandin production in mammals, reduced blood pressure, but also caused marked bradycardia. L-NAME, an inhibitor of all NO synthases, caused a small 10% rise in blood pressure, but cell-free hemoglobin (a potent NO scavenger and hypertensive agent in mammals) had no effect. Acute hypoxia caused a 15% fall in blood pressure, which was blocked by L-NAME and cell-free hemoglobin. Serum nitrite, a marker of NO production, rose with hypoxia, but not with L-NAME. Results suggest that NO is not an endothelial-derived vasodilator in the normoxic elasmobranch. The hypertensive effect of L-NAME may represent inhibition of NO production in the CNS and nerves regulating blood pressure. In acute hypoxia, there is a rapid up-regulation of vascular NO production that appears to be responsible for hypoxic vasodilation.

Localization of mRNAs encoding α and β subunits of soluble guanylyl cyclase in the brain of rainbow trout: comparison with the distribution of neuronal nitric oxide synthase

Detailed distribution of mRNAs encoding alpha and beta subunits of soluble guanylyl cyclase (sGC) was examined in the brain of rainbow trout by in situ hybridization. In addition, distribution of nitric oxide synthase (NOS) was mapped in adjacent parallel sections by neuronal NOS (nNOS) immunocytochemistry and NADPH-diaphorase (NADPHd) histochemistry. Following application of digoxigenin-labeled riboprobes for sGC alpha and beta subunit mRNAs, we found comparatively intense hybridization signals in the telencephalon, preoptic area, thalamus, hypothalamus, pretectum and tegmentum. Both nNOS immunocytochemistry and NADPHd histochemistry showed extensive distribution of nitroxergic neurons in various brain areas, although various degrees of dissociation of nNOS immunoreactivity (ir) and NADPHd staining were detected. In comparison with sGC subunit mRNAs, nNOS signals were more widely distributed in many neurons, including parvocellular neurons in the preoptic area, nucleus anterior tuberis in the hypothalamus, periventricular neurons in the optic tectum, most of the rhombencephalic neurons and pituitary cells. However, wide overlaps of sGC mRNA-containing neurons and nNOS-positive neurons were observed in the olfactory bulb, telencephalon, preoptic area, thalamus, hypothalamus, pretectum, optic tectum, tegmentum and cerebellum. The widespread overlapping in sGC subunit mRNAs and nNOS distribution suggests a role for sGC in various neuronal functions, such as processing of olfactory and visual signals and neuroendocrine function, possibly via NO/cGMP signaling in the brain of rainbow trout.

The early ontogeny of neuronal nitric oxide synthase systems in the zebrafish

To examine a putative role for neuronal nitric oxide synthase (nNOS) in early vertebrate development we investigated nNOS mRNA expression and cGMP production during development of the zebrafish Danio rerio. The nNOS mRNA expression in the central nervous system (CNS) and periphery showed a distinct spatio-temporal pattern in developing zebrafish embryo and young larvae. nNOS mRNA expression was first detected at 19 h postfertilisation (h.p.f.), in a bilateral subpopulation of the embryonic ventrorostral cell cluster in the forebrain. The number of nNOS mRNA-expressing cells in the brain slowly increased, also appearing in the ventrocaudal cell cluster from about 26 h.p.f., and in the dorsorostral and hindbrain cell cluster and in the medulla at 30 h.p.f. A major increase in nNOS mRNA expression started at about 40 h.p.f., and by 55 h.p.f. the expression constituted cell populations in differentiated central nuclei and in association with the proliferation zones of the brain, and in the medulla and retina. In parts of the skin, nNOS mRNA expression started at 20 h.p.f. and ended at 55 h.p.f. Between 40 and 55 h.p.f., nNOS mRNA expression started in peripheral organs, forming distinct populations after hatching within or in the vicinity of the presumptive swim bladder, enteric ganglia, and along the alimentary tract and nephritic ducts. Expression of nNOS mRNA correlated with the neuronal differentiation pattern and with the timing and degree of cGMP production. These studies indicate spatio-temporal actions by NO during embryogenesis in the formation of the central and peripheral nervous system, with possible involvement in processes such as neurogenesis, organogenesis and early physiology.

Expression of nitric oxide synthase in the preoptic-hypothalamo-hypophyseal system of the teleost Oreochromis niloticus

In the present study, we have analyzed the expression of nitric oxide synthase (NOS) in the preoptic-hypothalamo-hypophyseal system of the teleost Oreochromis niloticus. The assay for enzyme activity demonstrated that a constitutive NOS activity is present both in soluble and particulate fractions of the homogenates of diencephalons. Western blot analysis using an antibody against the N-terminus of human nNOS revealed two bands both in the supernatant and in the pellet. One band co-migrates at approximately 150 kDa with that detected in the rat cerebellum homogenates and presumably corresponds to neuronal NOS (nNOS) of mammals. The additional band, which migrates at approximately 180 kDa, might be attributed to an alternatively spliced nNOS isoform. Using NADPH diaphorase (NADPHd) histochemistry in combination with NOS immunohistochemistry, nNOS expression has been detected in preoptic nuclei, hypophysiotrophic nuclei of the ventral hypothalamus, and the pituitary gland. Various degrees of dissociation of NADPHd activity and nNOS immunoreactivity have been detected that could be attributed to the expression of different subtypes of nNOS in the preoptic/hypothalamo/hypophysial system of tilapia. In this paper, we also investigated the colocalization of nNOS with arginine-vasotocin (AVT) by means of immunolabeling of consecutive sections. Results suggest that NO may be colocalized with AVT in a subpopulation of neurosecretory neurons. Present findings suggest that nitric oxide (NO) is implicated in the modulation of hormone release in teleosts in a similar way to mammals.

Localization of the nitric oxide/cGMP signaling pathway-related genes and influences of morpholino knock-down of soluble guanylyl cyclase on medaka fish embryogenesis

To better understand the nitric oxide (NO) / cyclic GMP (cGMP) signaling pathway during embryogenesis, we examined the spatial and temporal expression pattern of the genes for neuronal nitric oxide synthase (nNOS), soluble guanylyl cyclase (soluble GC) subunit (OlGCS-alpha(1), OlGCS-alpha(2), and OlGCS-beta(1)), and cGMP-dependent protein kinase (cGK) I and II (cGK I and cGK II) in the medaka fish embryos. OlGCS-beta(1) and nNOS were expressed maternally and OlGCS-alpha(1), OlGCS-alpha(2),and cGK II were expressed zygotically. The zygotic expression of OlGCS-alpha(1) and cGK I was detected at stage 19, while that of OlGCS-alpha(2) was detected at stage 16. Whole-mount in situ hybridization showed that the expression of nNOS or cGK I was localized in tail bud, otic vesicles, thyroid, and brain ventricle, or in thymus, gill arch, and olfactory pits, respectively, and that of OlGCS-alpha(1), OlGCS-alpha(2), or OlGCS-beta(1) was dim and dispersed throughout the embryos. To clarify the "role of the NO/cGMP signaling pathway in embryogenesis, we examined the influences of morpholino antisense oligonucleotide of the soluble GC subunit gene (alpha(1)-MO, alpha(2)-MO or beta(1)-MO) on development of medaka fish embryos. Embryos injected with alpha(1)-MO or alpha(2)-MO mainly exhibited abnormalities in the central nervous system, including defects in the formation of forebrain, eye, and otic vesicles. alpha(2)-MO injection caused cell death at the tail bud of the embryos at stage 22, and beta(1)-MO injection inhibited the development of the embryos at late blastula. These results suggest that the NO/cGMP signaling pathway plays critical roles in early embryogenesis.

Expression of membrane-bound and soluble guanylyl cyclase mRNAs in embryonic and adult retina of the medaka fish Oryzias latipes

Localization of mRNAs for four membrane-bound guanylyl cyclases (membrane GCs; OlGC3, OlGC4, OlGC5, and OlGC-R2), three soluble guanylyl cyclase subunits (soluble GC; OlGCS-alpha(1), OlGCS-alpha(2), and OlGCS-beta(1)), neuronal nitric oxide synthase (nNOS), and cGMP-dependent protein kinase I (cGK I) was examined in the embryonic and adult retinas of the medaka fish Oryzias latipes by in situ hybridization. All of the membrane GC mRNAs were detected in the photoreceptor cells of the adult and embryonic retinas, but in different parts; the OlGC3 and OlGC5 mRNAs were expressed in the proximal part and the OlGC4 and OlGC-R2 mRNAs were expressed in the outer nuclear layer. The mRNA for nNOS was expressed in a scattered fashion on the inner side of the inner nuclear layer in the adult and embryonic retinas. The mRNAs (OlGCS-alpha(2) and OlGCS- beta(1)) of two soluble GC subunits (alpha(2) and beta(1)) were expressed mainly in the inner nuclear layer and the ganglion cell layer of the embryonic retina while the mRNAs of the soluble GC alpha(1) subunit and cGK I were not detected in either the adult or embryonic retina. These results suggest that NO itself and/or the cGMP generated by soluble GC (alpha(2)/beta(1) heterodimer) play a novel role in the neuronal signaling and neuronal development in the medaka fish embryonic retina in addition to the role played by phototransduction through membrane GCs in the adult and embryonic retinas.

Identification and localization of eight distinct hormone-producing cell types in the pituitary of male Atlantic halibut (Hippoglossus hippoglossus L.)

The eight distinct hormone-producing cell types in the adenohypophysis of male Atlantic halibut (Hippoglossus hippoglossus L.) were identified and localized using immunohistochemistry and in situ hybridization. Lactotropes either occupied most of the rostral pars distalis (RPD) or they were arranged in follicular structures located along the periphery of the RPD. Corticotropes were confined to a thin layer of RPD cells bordering the pars nervosa (PN). The somatotropes were arranged in multicellular layers bordering the highly convoluted PN penetrating the proximal pars distalis (PPD), while thyrotropes, scattered in small islets in between the somatotropes, were located in the centro-dorsal part of the PPD. Gonadotropes were found throughout the PPD. Immunoreactivity to glycoprotein-alpha and luteinizing hormone beta-subunit was also observed along the periphery of the pars intermedia (PI), indicating that a thin extension of the PPD surrounded the PI. In situ hybridization showed that follicle-stimulating hormone and luteinizing hormone were produced in distinct cells of the PPD. PI contained somatolactotropes bordering the highly convoluted PN, and melanotropes that showed positive immunostaining against both anti-alpha-melanocyte-stimulating hormone and anti-beta-endorphin. The general cellular organization was similar to that of other teleost fish. These results lay the basis for future investigations on Atlantic halibut pituitary physiology.

NADPH-diaphorase activity and nitric oxide synthase-like immunoreactivity colocalize in the electromotor system of four species of gymnotiform fish

The electric organ discharge (EOD) of gymnotiform electric fish is controlled by a well-characterized neural circuit in the brainstem and spinal cord. NADPH-diaphorase (NADPH-d) activity was previously found in phase-locking and/or rapidly firing neurons in the electromotor and electrosensory systems of Apteronotus leptorhynchus [Turner and Moroz, 1995]. These findings suggested that nitric oxide synthase (NOS) is expressed in these neurons and may regulate their precise, high frequency firing. We extended these results by examining the distribution of both NADPH-d activity and NOS-like immunoreactivity (NOS-lir) in the electromotor systems of four gymnotiform species that differ in the frequency and modulation of their EODs. NOS-lir colocalized with NADPH-d staining throughout the electromotor system, indicating that NADPH-d is a faithful indicator of NOS in this system. The distribution of NOS-lir and NADPH-d was similar in the electromotor systems of all four species in this study, with one exception: NOS and NADPH-d staining was consistently less intense in pacemaker and relay cells in Sternopygus macrurus, which produces low frequency EODs, than in the three other species that produce higher frequency EODs. This species difference in NOS expression in the pacemaker nucleus may be related to species differences either in EOD frequency or in modulations of the EOD (e.g., the jamming avoidance response). In Apteronotus species, NOS-lir and NADPH-d were concentrated in bands along the axons of their nerve-derived electric organs. These bands corresponded to regions surrounded by little or no staining with a Schwann cell-specific antibody, suggesting that the NOS-positive regions lie near nodes of Ranvier. In Sternopygus and Eigenmannia, the innervated, posterior membranes of muscle-derived electrocytes were more intensely labeled for NADPH-d and NOS than inexcitable portions of the membrane. Thus, in both muscle- and nerve-derived electric organs, NOS is concentrated near excitable membranes. These results indicate that NOS is well-positioned within the electromotor system to regulate the frequency, precision, amplitude, and waveform of EODs.

Nitric oxide and neuromodulation in the caudal neurosecretory system of teleosts

Although evidence exists that nitric oxide (NO) mediates neuroendocrine secretion in mammals, the involvement of NO in the neuroendocrine regulation of non-mammalian vertebrates has yet to be investigated in detail. The present review conveys several recent data, suggesting that NO plays a modulatory role in the caudal neurosecretory system (CNSS) of teleosts. The presence and distribution of neuronal NO synthase (nNOS) was demonstrated in the CNSS of the Nile tilapia Oreochromis niloticus by means of NADPHd histochemistry, NOS immunohistochemistry, NOS immunogold electron microscopy, the citrulline assay for NOS activity and Western blot analysis. NO production by the caudal spinal cord homogenates was also evaluated by the oxyhemoglobin assay. On the whole, these findings indicate that caudal neurosecretory cells express NOS enzymes and presumably produce NO as a cotransmitter. Moreover, the comparison of the nNOS distribution with that of urotensins I and II (UI and UII) suggests that neurosecretory Dahlgren cells belong to two different functional subpopulations: a population of UI/UII secreting nitrergic neurons and a population of non-nitrergic neurons, which principally secrete UII. These results implicate NO as a putative modulator of the release of urotensins from the neurosecretory axon terminals. Therefore, like in mammals, NO appears to influence neuroendocrine secretion in teleosts.

Nitric oxide synthase sequences in the marine fish Stenotomus chrysops and the sea urchin Arbacia punctulata, and phylogenetic analysis of nitric oxide synthase calmodulin-binding domains

The phylogenetic distribution and structural diversity of the nitric oxide synthases (NOS) remain important and issues that are little understood. We present sequence information, as well as phylogenetic analysis, for three NOS cDNAs identified in two non-mammalian species: the vertebrate marine teleost fish Stenotomus chrysops (scup) and the invertebrate echinoderm Arbacia punctulata (sea urchin). Partial gene sequences containing the well-conserved calmodulin (CaM)-binding domain were amplified by RT-PCR. Identical 375-bp cDNAs were amplified from scup brain, heart, liver and spleen; this sequence shares 82% nucleic acid and 91% predicted amino acid identity with the corresponding region of human neuronal NOS. A 387-bp cDNA was amplified from sea urchin ovary and testes; this sequence shares 72% nucleic acid identity and 65% deduced amino acid identity with human neuronal NOS. A second cDNA of 381 bp was amplified from sea urchin ovary and it shares 66% nucleic acid and 57% deduced amino acid identity with the first sea urchin sequence. Together with earlier reports of neuronal and inducible NOS sequences in fish, these data indicate that multiple NOS isoforms exist in non-mammalian species. Phylogenetic analysis of these sequences confirms the conserved nature of NOS, particularly of the calmodulin-binding domains.

Regional distribution of nitric oxide synthase and NADPH-diaphorase activities in the central nervous system of teleosts

Neuronal nitric oxide synthase (nNOS) and NADPH-diaphorase activities were investigated in discrete areas of the central nervous system of goldfish and brown trout. Both species showed a similar distribution pattern of nNOS activity with regional differences in all examined areas. Telencephalon and hypothalamus showed the highest nNOS values, while in the goldfish cerebellum and its valvula nNOS was not detectable. In both species, NADPH-diaphorase activity showed a lower regional variability, compared to nNOS. The highest activity was measured in the olfactory bulbs where, conversely, low levels of nNOS activity were present. The non close correspondence between NOS and NADPH-diaphorase activities confirms the discrepancies indicated by morphological data. Western blot analysis revealed the presence of a nNOS isoform of about 150 kDa mol. wt. corresponding to that of mammals. The pattern of nNOS expression in the considered brain regions of the goldfish and trout was comparable to the levels of the nNOS activity.

Identification and distribution of nitric oxide synthase in the brain of adult zebrafish

Nitric oxide (NO) is proposed to be involved in developmental and plastic processes. We investigated the presence and distribution of nitric oxide synthase (NOS) in the zebrafish (Danio rerio) using molecular and histochemical techniques. A partial gene sequence corresponding to the neuronal NOS isoform (nNOS) was identified, and in situ hybridization revealed cellular nNOS mRNA expression throughout the brain of adult zebrafish, distributed in distinct central nuclei and in proliferation zones. NOS immunoreactivity and nicotinamide adenine dinucleotide phosphate diaphorase activity partly coincided with the nNOS mRNA expression, however was present also in additional neuronal and non-neuronal cell types. The results indicate the occurrence of different NOS isoforms in the adult brain, of which nNOS may participate in neurotransmission, and in mechanisms related to the continuous growth and neuronal plasticity of the teleost brain.