Localization of the neuronal form of nitric oxide synthase (bNOS) in the diencephalon and pituitary gland of the catfish, Synodontis multipunctatus: an immunocytochemical study

Nitric oxide mediated kisspeptin regulation of steroidogenesis and gametogenesis in the catfish, Clarias batrachus

Nitric oxide (NO) is a gaseous molecule that regulates various reproductive functions. It is a well-recognized regulator of GnRH-FSH/LH-sex steroid secretion in vertebrates including fish. Kisspeptin is a recently discovered neuropeptide which also regulates GnRH secretion. Nitrergic and kisspeptin neurons are reported in close physical contact in the mammalian brain suggesting their interactive role in the release of GnRH. The existence of kisspeptin and NOS is also demonstrated in vertebrate gonads, but information on their reciprocal relation in gonads, if any, is obscure. Therefore, attempts were made to evaluate the functional reciprocal relation between nitric oxide and kisspeptin in the catfish gonads, if any, by administering the nitric oxide synthase (NOS) inhibitor, L-NAME {N(G)-nitro-L-arginine methyl ester}, which reduces NO production, and kisspeptin agonist (KP-10) and assessing their impacts on the expressions of kisspeptin1, different NOS isoforms, NO and steroid production in the gonadal tissue. The results revealed that L-NAME suppressed the expression of kiss1 in gonads of the catfish establishing the role of NO in kisspeptin expression. However, KP-10 increased the expression of all the isoforms of NOSs (iNOS, eNOS, nNOS) and concurrently NO and steroids in the ovary and testis. In vitro studies also indicate that kisspeptin stimulates the production of NO and estradiol and testosterone levels in the gonadal explants and medium. Thus, in vivo results clearly suggest a reciprocal interaction between kisspeptin and NO to regulate the gonadal activity of the catfish. The in vitro findings further substantiate our contention regarding the interactive role of kisspeptin and NO in gonadal steroidogenesis.

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.

Role of catecholamines and nitric oxide on pigment displacement of the chromatophores of freshwater snakehead teleost fish, Channa punctatus

We are reporting for the first time that the catecholamines (adrenaline and noradrenaline) inhibit the effect of nitric oxide (NO) on melanosome dispersion in freshly isolated scales of the freshwater snakehead fish, Channa punctatus. We studied the effect of NO and catecholamines on the pigment displacement by observing the changes in the melanophore index. The scales when treated with solution containing NO donor sodium nitroprusside (SNP) showed dispersion of melanosomes, whereas NO synthase blocker N-omega-Nitro-L-arginine suppresses this action of SNP. Treatment with adrenaline and noradrenaline on the isolated scales caused aggregation of melanosomes. Scales treated with solution containing catecholamines and SNP resulted in aggregation of melanosomes suggesting that catecholamines mask the effect of SNP. These results suggest that the catecholamines are inhibiting the effect of NO and causing the aggregation of the melanosomes may be via surface receptors.

Nitric oxide inhibited the melanophore aggregation induced by extracellular calcium concentration in snakehead fish, Channa punctatus

We studied the role of nitric oxide (NO) and extra-cellular Ca(2+) on the melanophores in Indian snakehead teleost, Channa punctatus. Increase of Ca(2+) level in the external medium causes pigment aggregation in melanophores. This pigment-aggregating effect was found to be inhibited when the external medium contained spontaneous NO donor, sodium nitro prusside (SNP) at all the levels of concentration tested. Furthermore, it has been observed that SNP keeps the pigment in dispersed state even after increasing the amount of Ca(2+). In order to test whether NO donor SNP causes dispersion of pigments or not is checked by adding the inhibitor of nitric oxide synthase, N-omega-Nitro-L-arginine (L-NNA) in the medium. It has been noted that the inhibitor L-NNA blocked the effect of NO donor SNP causing aggregation of pigments. In that way NO is inhibiting the effect of extracellular Ca(2+), keeping the pigment dispersed.

The ontogeny of Mauthner cells in the brain of Labeo rohita as revealed by NADPH-d and nNOS immunohistochemistry

Nitric oxide (NO) is well demonstrated to act as a neuronal messenger in neurotransmission in vertebrate animals. We are for the first time reporting nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and neuronal nitric oxide synthase (nNOS) in the Mauthner cells (MC) of the carp, Labeo rohita brain during post-embryonic development. The NADPH-d and nNOS-positive MC were seen intensely labeled in all young stages on either side of the ventricle in the hindbrain. In adult, these cells were moderately stained with NADPH-d but not with nNOS. Further, the morphometeric analysis of the staining intensities showed the decrease in the optical densities of the MC as the development progresses. The gradual increase in cell diameter was evident from hatchling to adult (18-22 μm to 98-106 μm), while the nuclear diameter remained nearly unchanged (10-12 μm to 18-20 μm). In fishes, MC are known to be involved in the control of startle response. The NADPH-d enzymatic activity seen in the MC of early development may play an important role to activate the startle response to fast escape from the predators and unfavorable environmental conditions to survive during early phase of life. As nNOS is absent in the adult MC, we assume that the adult MC express a different splice variant of nNOS which is detected by NADPH-d, but not by the currently used antibody. Hence, the presence of nNOS (enzyme required for the synthesis of NO) in the MC suggests the involvement of NO in neurotransmission to generate the startle response rapidly. Furthermore, we also suggest that NO may be involved in other aspects such as pathway finding, neuronal cell migration, signaling and refinement during the development of the brain of L. rohita.

Seasonality in expression and distribution of nitric oxide synthase isoforms in the testis of the catfish, Clarias batrachus: role of nitric oxide in testosterone production

Nitric oxide (NO) is a well-recognized versatile signaling molecule. It is produced by catalytic action of nitric oxide synthase (NOS) on L-arginine in a variety of animal tissues. Existence of different isoforms of NOS has been shown in mammalian testis, but report on their presence in the testis of ectothermic vertebrates is non-existent. This study demonstrates the differential expressions of two isoforms of nitric oxide synthase (neuronal-nNOS and inducible-iNOS) like molecules in different cell types in the testis of seasonally breeding catfish, Clarias batrachus through immunohistochemistry. Positive immunoprecipitation of nNOS and iNOS like molecules were detected in germ cells as well as interstitial cells only in the recrudescing and fully mature fish. The immunoreactions differed in intensity and varied with changing reproductive status. Treatment of adult male fish with NO donor, sodium nitroprusside, and a NOS inhibitor, N-nitro-L-arginine methyl ester (L-NAME) increased and decreased the total nitrate and nitrite concentration in the testis, respectively. Sodium nitroprusside and L-NAME also induced simultaneous decline and rise in the testicular testosterone level, respectively. These findings, thus, suggest that NOS isoforms are expressed variedly in different cell types in the testis of reproductively active fish. This investigation also suggests that NO inhibits testosterone production in the testis.

Neuronal nitric oxide synthase in the olfactory system, forebrain, pituitary and retina of the adult teleost Clarias batrachus

Immunocytochemical application of antibodies against nNOS to the brain sections of Clarias batrachus revealed intense immunoreactivity in several olfactory receptor neurons (ORNs), in their axons over the olfactory nerve, and terminals in the olfactory glomeruli. Several basal cells in the olfactory epithelium showed NOS immunoreactivity. Application of post-embedding immunoelectron microscopy showed nNOS labeled gold particles in apical cilia, dendrites and soma of the ORNs and also in the axon terminals in the glomeruli of the olfactory bulb. nNOS containing fibers were also encountered in the medial olfactory tracts (MOTs). Bilateral ablation of the olfactory organ resulted in total loss of nNOS immunoreactivity in the fascicles of the olfactory nerve layer and also in the MOT. nNOS immunoreactivity was seen in several cells of the nucleus preopticus (NPO) and their axons that innervate the pituitary gland. Some cells in the floor of the tuberal area were stained positive with nNOS antibodies. nNOS immunolabeled cells were seen in all the three components of the pituitary gland with light as well as post-embedding immunoelectron microscopy. While several nNOS immunoreactive fibers were seen in rostral pars distalis, a much limited fiber population was seen in the proximal pars distalis. In addition, conspicuous immunoreactivity was noticed in some ganglion cells in the retina and in some fibers of the optic nerve traceable to the optic tectum. The NO containing system in this fish appears to be similar to that in other fishes.

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.

Co-localization of neuronal nitric oxide synthase with arginine–vasotocin in the preoptic–hypothalamo–hypophyseal system of the teleost Oreochromis niloticus

This study provides evidence that, in the preoptic-hypothalamo-hypophysial system of the teleost Oreochromis niloticus, several sub-populations of arginine-vasotocin (AVT)-producing neurons and neurosecretory fibers terminals express neuronal nitric oxide synthase (nNOS)-like molecules. The co-localization between nNOS and AVT was demonstrated by means of double immunofluorescence staining with the confocal microscope. This study is the first to provide evidence that nNOS may be co-localized with AVT in neurons of a non-mammalian vertebrate.

Nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase activity in the brain of a cichlid fish, with remarkable findings in the entopeduncular nucleus: a histochemical study

In the entire brain of the African cichlid fish, Haplochromis burtoni, nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity was visualized histochemically in fewer nuclei compared to other teleost fish. Intensively labeled perikarya were found in the ventral hypothalamic area, the nucleus of the medial longitudinal fascicle, the nucleus of the midbrain tegmentum, the nucleus of the lateral longitudinal fascicle, the trigeminal motor nucleus and the octavolateral area. Compared to other NADPH-d labeled nuclei in the brain, we saw an unusual localization of NADPH-d activity in the rostral, dorsal, ventral and caudal part of the entopeduncular nucleus. Additionally, some isolated perikarya of different morphological appearance were found at the levels of the preglomerular nucleus, the diffuse nucleus of the lateral torus and the lateral longitudinal fascicle. A widespread distribution of labeled fibers was identified throughout the brain. The remarkable NADPH-d activity, particularly in the entopeduncular nucleus, differs significantly from the existing data on other teleosts. Taking into account the sensory functions of the entopeduncular nucleus described in other vertebrates, the massive NADPH-d activity in this nucleus may indicate an important role of NADPH-d in the modulation of sensory functions.