Re-evaluating NADPH-diaphorase histochemistry as an indicator of nitric oxide synthase: an examination of the olfactory system of coho salmon (Oncorhynchus kisutch)

Expression and localization of neuronal nitric oxide synthase in the brain and sensory tissues of the African cichlid fish Astatotilapia burtoni

Nitric oxide (NO) produced by the enzyme neuronal nitric oxide synthase serves as an important neurotransmitter in the central nervous system that is involved in reproductive regulation, learning, sensory processing, and other forms of neural plasticity. Here, we map the distribution of nnos-expressing cells in the brain and retina of the cichlid fish Astatotilapia burtoni using in situ hybridization. In the brain, nnos-expressing cells are found from the olfactory bulbs to the hindbrain, including within specific nuclei involved in decision-making, sensory processing, neuroendocrine regulation, and the expression of social behaviors. In the retina, nnos-expressing cells are found in the inner nuclear layer, presumably in amacrine cells. We also used quantitative PCR to test for differences in nnos expression within the eye and olfactory bulbs of males and females of different reproductive states and social statuses. In the eye, males express more nnos than females, and socially dominant males express more nnos than subordinate males, but expression did not differ among female reproductive states. In the olfactory bulbs, dominant males had greater nnos expression than subordinate males. These results suggest a status-specific function for NO signaling in the visual and olfactory systems that may be important for sensory perception related to mating or territorial interactions to maintain the social hierarchy. The widespread distribution of nnos-expressing cells throughout the cichlid brain is similar to that in other teleosts, with some conserved localization patterns across vertebrates, suggesting diverse functions for this important neurotransmitter system.

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.

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.

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.

Direct single cell determination of nitric oxide synthase related metabolites in identified nitrergic neurons

The biochemical characterization of individual nitrergic (NO releasing) neurons is a non-trivial task both in vertebrate and invertebrate preparations. In spite of numerous efforts, there are limited data related to intracellular concentrations of essential metabolites involved in NO synthesis and degradation. This situation creates controversies in both identification of nitrergic neurons and the selection of reliable reporters of NOS activity in heterogeneous cell populations. We take advantage of identified neurons from the pulmonate mollusc Lymnaea stagnalis to perform direct single cell microanalysis of intracellular concentrations of the major nitric oxide synthase (NOS) related metabolites such as arginine, citrulline, argininosuccinate, NO(2)(-),and NO(3)(-). Capillary electrophoresis protocols have been developed to quantitate levels of these metabolites in single identified neurons from the buccal, cerebral, and pedal ganglia using laser-induced fluorescence and conductivity detection. The limits of detection (LODs) for arginine (Arg) and citrulline (Cit) are 84 amol (11nM) and 110 amol (15 nM), respectively, and LODs for NO(2)(-)and NO(3)(-) are <200 amol (<10nM) each. We report that intracellular concentrations of NOS related metabolites are in the millimolar range and less than 1% of a single cell is required for microchemical analysis. From four cell types tested, only the esophageal motoneuron B2 contains active NOS, and they also contain surprisingly high nitrite levels (up to 5mM) compared to other neurons tested (peptidergic B4, dopaminergic RPeD1, and serotonergic CGC). These B2 neurons also exhibit an Arg/Cit ratio susceptible to the selective NOS inhibitor l-iminoethyl-N-ornithine whereas others neurons do not even though they all may contain NOS transcripts. On the contrary, we found that absolute concentrations of other NOS related metabolites including nitrates are not reliable markers of NOS activity and demonstrate the need for multiple assays for NOS activity.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

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.

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.

Neuronal nitric oxide synthase in the olfactory system of an adult teleost fish Oreochromis mossambicus

The aim of the present study is to explore the distribution of nitric oxide synthase in the olfactory system of an adult teleost, Oreochromis mossambicus using neuronal nitric oxide synthase (nNOS) immunocytochemistry and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry methods. Intense nNOS immunoreactivity was noticed in several olfactory receptor neurons (ORNs), in their axonal extensions over the olfactory nerve and in some basal cells of the olfactory epithelium. nNOS containing fascicles of the ORNs enter the bulb from its rostral pole, spread in the olfactory nerve layer in the periphery of the bulb and display massive innervation of the olfactory glomeruli. Unilateral ablation of the olfactory organ resulted in dramatic loss of nNOS immunoreactivity in the olfactory nerve layer of the ipsilateral bulb. In the olfactory bulb of intact fish, some granule cells showed intense immunoreactivity; dendrites arising from the granule cells could be traced to the glomerular layer. Of particular interest is the occurrence of nNOS immunoreactivity in the ganglion cells of the nervus terminalis. nNOS containing fibers were also encountered in the medial olfactory tracts as they extend to the telencephalon. The NADPHd staining generally coincides with that of nNOS suggesting that it may serve as a marker for nNOS in the olfactory system of this fish. However, mismatch was encountered in the case of mitral cells, while all are nNOS-negative, few were NADPHd positive. The present study for the first time revealed the occurrence of nNOS immunoreactivity in the ORNs of an adult vertebrate and suggests a role for nitric oxide in the transduction of odor stimuli, regeneration of olfactory epithelium and processing of olfactory signals.

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

The distribution of the neuronal form of nitric oxide synthase (bNOS) was investigated in the brain and pituitary gland of the catfish, Synodontis multipunctatus. Immunoreactive neurons were found mainly in the nucleus praeopticus periventricularis, the parvocellular and supraoptic subdivisions of the nucleus praeopticus, the nucleus recessus lateralis and the nucleus recessus posterioris. In addition, some scattered bNOS labeled somata were noted in the dorsal hypothalamic area. A few positive cells in the adenohypophysis and some reactive fibers in the pituitary stalk were also seen. Our results are compatible with the notion that the cells expressing bNOS in the diencephalon and hypophysis are involved in the control of hormone regulation. Moreover, the presence of bNOS positive cells in the rostral pars distalis of the pituitary gland supports a role of nitric oxide in osmoregulation.

NADPH diaphorase activity in olfactory receptor neurons and their axons conforms to a rhinotopically-distinct dorsal zone of the hamster nasal cavity and main olfactory bulb

NADPH diaphorase histochemical protocols were optimized for the histochemical labeling of olfactory receptor neurons (ORNs) in the nasal cavity and their axon terminals in glomeruli of the main olfactory bulb (MOB) in the Syrian hamster. This labeling was then used to map and quantify the spatial distribution of ORNs and their central projections. Diaphorase-positive ORNs were found to be rhinotopically restricted to dorsal-medially situated segments of sensory mucosa associated with central air channels in the nose, together constituting about 25% of the total receptor sheet. This topography closely resembles the zonal expression patterns of putative odorant receptor genes and cell surface glycoconjugates in the nose. Moreover, the projections of ORNs in the diaphorase-positive dorsal/central zone were found to expand onto the entire dorsal half of the MOB, consistent with spatial patterns discerned in retrograde tract-tracing studies. These boundaries indicate that dorsal/central zone ORNs project to a disproportionately larger region of the MOB than do those in the more ventral/peripheral zones. The demonstration of NADPH diaphorase activity in ORNs is inconsistent with the expression of the best-known NADPH-dependent enzymes, such as nitric oxide synthase (neuronal and endothelial isoforms) and NADPH cytochrome P450 oxidoreductase. Understanding the spatial patterning of histochemical labeling in ORNs should facilitate the biochemical identification of this diaphorase.