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

Inducible Nitric Oxide Synthase/Nitric Oxide System as a Biomarker for Stress and Ease Response in Fish: Implication on Na+ Homeostasis During Hypoxia

The cellular and organismal response to stressor-driven stimuli evokes stress response in vertebrates including fishes. Fishes have evolved varied patterns of stress response, including ionosmotic stress response, due to their sensitivity to both intrinsic and extrinsic stimuli. Fishes that experience hypoxia, a detrimental stressor that imposes systemic and cellular stress response, can evoke disturbed ion homeostasis. In addition, like other vertebrates, fishes have also developed mechanisms to recover from the impact of stress by way of shifting stress response into ease response that could reduce the magnitude of stress response with the aid of certain neuroendocrine signals. Nitric oxide (NO) has been identified as a potent molecule that attenuates the impact of ionosmotic stress response in fish, particularly during hypoxia stress. Limited information is, however, available on this important aspect of ion transport physiology that contributes to the mechanistic understanding of survival during environmental challenges. The present review, thus, discusses the role of NO in Na⁺ homeostasis in fish particularly in stressed conditions. Isoforms of nitric oxide synthase (NOS) are essential for the synthesis and availability of NO at the cellular level. The NOS/NO system, thus, appears as a unique molecular drive that performs both regulatory and integrative mechanisms of control within and across varied fish ionocytes. The activation of the inducible NOS (iNOS)/NO system during hypoxia stress and its action on the dynamics of Na⁺/K⁺-ATPase, an active Na⁺ transporter in fish ionocytes, reveal that the iNOS/NO system controls cellular and systemic Na⁺ transport in stressed fish. In addition, the higher sensitivity of iNOS to varied physical stressors in fishes and the ability of NO to lower the magnitude of ionosmotic stress in hypoxemic fish clearly put forth NO as an ease-promoting signal molecule in fishes. This further points to the signature role of the iNOS/NO system as a biomarker for stress and ease response in the cycle of adaptive response in 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.

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.

ACTH-stimulated cortisol release from head kidney of rainbow trout is modulated by glucose concentration

To assess the hypothesis that cortisol release in rainbow trout is modulated by glucose levels, we first evaluated cortisol release [basal and adrenocorticotropic hormone (ACTH)-regulated] by head kidney tissue superfused with medium reflecting hypoglycaemic, normoglycaemic or hyperglycaemic conditions. Next, cortisol release from head kidney fragments in static incubations was assessed in parallel with changes in parameters related to cortisol synthesis (mRNA abundance of StAR, P450scc, 3βHSD and 11βH) and the GK-mediated glucosensing mechanism (levels of glycogen and glucose, activities of GK, GSase and PK, and mRNA levels of GK, GLUT-2, Kir6.x-like and SUR-like). We then evaluated the effects of two inhibitors of glucose transport, cytochalasin B and phlorizin, on cortisol production and glucosensing mechanisms. The ACTH-induced release of cortisol proved to be modulated by glucose concentration such that increased release occurs under high glucose levels, and decreased ACTH-stimulated cortisol release occurs when glucose transport is inhibited by cytochalasin B. The release of cortisol can be associated with increased synthesis as enhanced mRNA abundance of genes related to cortisol synthesis was also noted in high glucose medium. Specific GK immunoreactivity in the cortisol-producing cells (not in chromaffin cells) further substantiates GK-mediated glucosensing in cortisol production. In contrast, no changes compatible with those of glucose levels and cortisol release/synthesis in the presence of ACTH were noted for any other putative glucosensor mechanisms based on LXR, SGLT-1 or Gnat3. These combined results are the first evidence for a mechanism in fish linking the synthesis and release of a non-pancreatic hormone like cortisol with circulating glucose levels. The relationship was evident for the regulated (ACTH-dependent) pathway and this suggests that under acute stress conditions glucose is important for the regulation of cortisol synthesis and release.

Nitric oxide: An autocrine regulator of Leydig cell steroidogenesis in the Asian catfish, Clarias batrachus

Nitric oxide has been recognized as an important inter- and intra-cellular modulator of testicular steroidogenesis in higher vertebrates with conflicting results. Moreover, its role in regulation of testicular steroidogenesis in ectothermic vertebrates is non-existent. The present study was, therefore, undertaken to examine whether Leydig cells of a freshwater catfish, Clarias batrachus produce nitric oxide (NO), if so, does it regulate its steroidogenic activity? The purified Leydig cells were stained histochemically for NADPH-diaphorase (NADPH-d) activity, and immunocytochemically for neuronal and inducible nitric oxide synthase (nNOS and iNOS) like molecules. Leydig cells were also incubated with NOS inhibitor, N-nitro-l-arginine methyl ester (l-NAME), and NO donor, sodium nitroprusside (SNP). NO and testosterone released in incubation medium were analyzed. A distinct positive NAPDH-d staining was observed in Leydig cells. These cells also exhibited immunoprecipitation of variable intensity with nNOS and iNOS antibodies. Further, l-NAME treatment caused significant suppression in NO production and elevation in testosterone secretion by Leydig cells. On the contrary, exposure of Leydig cells to SNP resulted in increased NO production with concomitant decline in testosterone level. Thus, the present study reports NO production by Leydig cells in fish for the first time, which appears to inhibit its own androgen production.

The interactive effects of hypoxia and nitric oxide on catecholamine secretion in rainbow trout (Oncorhynchus mykiss)

Experiments were performed to test the hypothesis that exposure of rainbow trout to repetitive hypoxia would result in a decreased capacity of chromaffin cells to secrete catecholamines owing to increased production of nitric oxide(NO), a potent inhibitor of catecholamine secretion. A partial sequence of trout neuronal nitric oxide synthase (nNOS) was cloned and its mRNA was found to be present in the posterior cardinal vein (PCV), the predominant site of chromaffin cells in trout. Using heterologous antibodies, nNOS and endothelial NOS (eNOS) were localized in close proximity to the chromaffin cells of the PCV.

Exposure of trout to acute hypoxia (5.33 kPa for 30 min) in vivoresulted in significant increases in plasma catecholamine and NO levels. However, after 4 days of twice-daily exposures to hypoxia, the elevation of plasma catecholamine levels during hypoxia was markedly reduced. Associated with the reduction in plasma catecholamine levels during acute hypoxia was a marked increase in basal and hypoxia-evoked circulating levels of NO that became apparent after 2-4 days of repetitive hypoxia. The capacity of the chromaffin cells of the hypoxia-exposed fish to secrete catecholamine was assessed by electrical stimulation of an in situ saline-perfused PCV preparation. Compared with control (normoxic) fish, the PCV preparations derived from fish exposed to repeated hypoxia displayed a significant reduction in electrically evoked catecholamine secretion that was concomitant with a marked increased in NO production. This additional rise in NO secretion in preparations derived from hypoxic fish was prevented after adding NOS inhibitors to the perfusate; concomitantly, the reduction in catecholamine secretion was prevented. The increased production of NO during hypoxia in vivo and during electrical stimulation in situ was consistent with significant elevations of nNOS mRNA and protein; eNOS protein was unaffected. These results suggest that the reduced capacity of trout chromaffin cells to secrete catecholamines after repeated hypoxia reflects an increase in the expression of nNOS and a subsequent increase in NO production during chromaffin-cell activation.

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.

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 and the control of catecholamine secretion in rainbow trout Oncorhynchus mykiss

An in situ saline-perfused posterior cardinal vein preparation was used to assess the role of nitric oxide (NO) in the regulation of basal and stimulus-evoked catecholamine secretion from rainbow trout Oncorhynchus mykiss chromaffin cells. Addition of the NO donor, sodium nitroprusside (SNP) to the inflowing perfusate abolished catecholamine secretion during electrical field stimulation, thereby establishing the potential for NO to act as a potent inhibitor of catecholamine release. A possible role for endogenously produced NO was established by demonstrating that stimulus-evoked (depolarizing levels of KCl or electrical field stimulation) catecholamine secretion was markedly stimulated in the presence of the nitric oxide synthase (NOS) inhibitors l-NAME and 7-NI. Although in vitro experiments demonstrated that catecholamine degradation was enhanced by NO in a dose-dependent manner, the dominant factor contributing to the reduction in catecholamine appearance in the perfusate was specific inhibition of catecholamine secretion. Subsequent experiments were performed to identify the NOS isoform(s) contributing to the inhibition of stimulus-evoked catecholamine secretion. Inducible NOS (iNOS; an enzyme that can be activated in the absence of Ca2+), although present in the vicinity of the chromaffin cells (based on mRNA measurements), does not appear to play a role because stimulus-evoked NO production was eliminated during perfusion with Ca2+-free saline. The potential involvement of endothelial NOS (eNOS) was revealed by showing that hypoxic perfusate evoked NO production and corresponded with an inhibition of stimulus-evoked catecholamine secretion; chemical removal of the endothelium (using saponin) prevented the production of NO during hypoxia. However, because removal of the endothelium did not affect NO production during electrical field stimulation, it would appear that the neuronal form of NOS (nNOS) is the key isoform modulating catecholamine secretion from trout chromaffin cells.

Role of nitric oxide in larval and juvenile fish

Fish are known to express the three isoforms of nitric oxide synthase (NOS), the constitutive forms endothelial or eNOS, neuronal or nNOS and the inducible form iNOS. Most studies in fish have focussed on possible roles for NO in cardiovascular physiology although there has been recent attention on the role of nNOS in embryonic development. However compared to mammalian studies there have been relatively few studies on effects of nitric oxide (NO) on fish. Studies on heart and blood vessel preparations from various fish species appear to show results specific to the species or to the particular preparation. Possible roles of NO in the in vivo biology of adult fish or larval fish have received little attention. This article reviews effects of nitric oxide on cardiovascular physiology in fish with special emphasis on larval fish. It introduces some experimental work on possible signaling pathways in larval fish and introduces the possibility that NO could be an important environmental influence for some aquatic organisms. In higher vertebrates LPS (lipopolysaccharide) is known to activate the cytokine signaling system and stimulate increased expression of iNOS and increased production of NO, but this remains less investigated in fish. The effects of LPS on cardiovascular and osmoregulatory physiology of larval and juvenile salmonids are discussed and a possible role of NO in stress-induced drinking is suggested.

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.

The development of adrenal homolog of rainbow trout Oncorhynchus mykiss: an immunohistochemical and ultrastructural study

In this work we describe the adrenal homolog of the rainbow trout Oncorhynchus mykiss during development. At the histological level, the interrenal primordium is clearly evident in larvae 25 days after fertilization (dpf), and the immunohistochemical reactions for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyltransferase (PNMT), which mark the chromaffin cells, appear as early as 27 dpf. Both reactions are evident in cells localized in the head kidney and in some, probably migrating, cells close to the notochord. In 27-dpf larvae, the ultrastructural analysis shows the presence of the interrenal cells with mitochondria with tubulovesicular cristae, typical of steroidogenic cells, sometimes surrounded by smooth endoplasmic reticulum (SER) cisternae, indicating that in this stage the cells have the capacity for steroid synthesis and secretion. In the same stage the chromaffin cells are characterized by few and small membrane-bound granules containing cores of heterogeneous electron density. Both types of cells show large nuclei, numerous free or clumped ribosomes, developed rough endoplasmic reticulum (RER), and scarce SER. Rare nerve endings contacting chromaffin cells are present. In the subsequent developmental stages, a further differentiation of both types of cells is evidenced by modifications of cell organelles as mitochondria, chromaffin granules, RER, SER, and so on. A clear discrimination of the two types of catecholamine-containing cells, adrenaline and noradrenaline cells, is evident only 5 days after hatching. The presence of different interrenal cell types in larvae at 5 and 10 days after hatching probably indicates the activation of a physiological cellular cycle. The immunohistochemical and ultrastructural results are compared with those obtained by other authors in the same and other vertebrate species.

Survey of the Adrenal Homolog in Teleosts

The adrenal homolog of teleosts is not a compact organ as the adrenal glands of most vertebrates but is composed by aminergic chromaffin and interrenal steroidogenic cells located mostly inside the head kidney that, in this taxon, generally has a hematopoietic function. The two tissues can be mixed, adjacent, or completely separated and line the endothelium of the venous vessels or are located in close proximity. The chromaffin cells in some species are also present in the posterior kidney. Histological and ultrastructural work revealed cytological peculiarities of both types of cells as compared to those of other vertebrate species. In particular, the interrenal ones can show some variations in ultrastructure depending on sex, time of the year, and relation to stress events. A periodic renewal of the whole gland tissue is also sustained by some studies. Research regarding development is scanty as compared to mammals and most studies go back to the early years of the past century. The adrenal homolog of teleosts is under hormonal and neuronal control. Moreover, local paracrine interactions may play an important role in modulating a system involved in stress response and osmoregulation. Most previous studies involved a few species with the object of intensive rearing for commercial purposes; in fact cortisol, the main hormone secreted by the interrenal cells, can also influence reproduction and growth. This review summarizes data from morphocytological work and refers to other excellent reviews regarding physiology. Some of the results are compared to data available from other fishes and vertebrate classes with the aim of including them in an evolutionary and environmental framework.