Neuronal and glial cell types revealed by NADPH-diaphorase histochemistry in the retina of a teleost fish, the grass goby (Zosterisessor ophiocephalus, Perciformes, Gobiidae)

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.

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.

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.

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.

Carnosine-like immunoreactivity in neurons of the brain of an advanced teleost, the gray mullet (Chelon labrosus, Risso)

The distribution of the dipeptide carnosine (beta-alanyl-L-histidine) and the related dipeptides anserine and homocarnosine has been studied by biochemical methods and immunohistochemistry in the brain of mammals and other tetrapods. These studies have indicated the presence of these dipeptides mainly in glial cells and in some neurons (olfactory receptor neurons and certain putative migrating neurons and neuroblasts). Here, we used immunohistochemistry with a polyclonal anti-carnosine antibody and the streptavidin-avidin method to study for the first time the distribution of carnosine/carnosine-related dipeptides in the brain of a teleost fish (the gray mullet Chelon labrosus). In order to assess the neuronal nature of carnosine-immunoreactive cells, we also used double immunofluorescence methods with antibodies to carnosine and to the neuronal protein HuC/D. The results obtained show that carnosine and/or related dipeptides are present in neurons of various brain regions. The carnosine-like-immunoreactive neuronal populations of the optic tectum and cerebellum are described in detail. In the optic tectum, immunoreactivity to carnosine/carnosine-related dipeptides is present in neurons of the stratum album and the stratum griseum centrale. In the cerebellum, immunoreactivity to these dipeptides is localized in Purkinje cells, in putative Golgi and stellate cells, and in many nerve fibers. Carnosine-like immunoreactive cells in mullet brain seem to be specific types of neurons, in line with previous results in a urodele but at variance with previous results in mammals, reptiles and anurans.

Isolation and characterization of two teleost melanopsin genes and their differential expression within the inner retina and brain

Melanopsin is a newly discovered photopigment that is believed to be involved in the regulation of circadian rhythms in tetrapods. Here we describe the characterization of the first two teleost melanopsins (opn4a and opn4b) isolated from Atlantic cod (Gadus morhua). These two teleost genes belong to a subgroup of melanopsins that also include members from Xenopus, chicken, and Takifugu. In situ hybridization revealed that opn4a and opn4b are differentially expressed within the retina and brain. In the larval and adult retina, both melanopsins are expressed in a subset of cells in the inner retina, resembling amacrine and ganglion cells. In addition, opn4a is expressed in the horizontal cells, indicating a separate task for this gene. In the brain, the two melanopsins are separately expressed in two major retinal and extraretinal photosensitive integration centers, namely, the suprachiasmatic nucleus (opn4a) and the habenula (opn4b). The expression of opn4a in the suprachiasmatic nucleus in cod is similar to the melanopsin expression found in Xenopus. This suggests a conserved role for this opsin and an involvement in mediation of nonvisual photoreceptive tasks, such as entraining circadian rhythms and/or hypophysiotrophic systems. The differential expression of opn4b in the habenula suggests that this gene plays a role similar to that of opn4a, in that it is also situated in an area that integrates photic inputs from the pineal as well as other brain regions. Thus, the habenula may be an additional region that mediates photic cues in teleosts.

Choline acetyltransferase is found in terminals of horizontal cells that label with GABA, nitric oxide synthase and calcium binding proteins in the turtle retina

In this study, we discriminated the various types of horizontal cell in the turtle retina on their content of neuroactive substances. Double label immunocytochemistry was performed on sectioned and wholemount retina using antisera to neural- and endothelial-nitric oxide synthase (nNOS, and eNOS), calretinin (CR), calbindin (CB), gamma-aminobutyric acid (GABA) and choline acetyltransferase (ChAT). H1 cells and their axon terminals label with CR, CB and GABA. Only H1 axon terminals label with eNOS. H2 cells contain CB, CR, nNOS and GABA maybe in their dendrites. H3 cells label only with nNOS. The localization of nNOS in the H2 and H3 cells is a novel finding. None of these antibodies labels H4 cells. The photoreceptor subtypes have been differentiated by different intensity of labeling with CB. The accessory member of the double cone is less intensely labeled with CB than the principal member and rods and blue cones do not appear to label at all. ChAT-IR is located in terminal boutons of H1 and H2 horizontal cells and H1 axon terminals and these boutons contact rods and all spectral types of cones. Clearly, GABA is present in H1 horizontal cells and may be used in neurotransmission between horizontal cells and possibly for feedback pathways to photoreceptors. The evidence of nNOS immunoreactivity in H2 and H3 horizontal cells, combined with available physiological evidence, suggests that NO may be involved in electrical coupling and/or modulation of synaptic input to these types of cells. Furthermore, our results raise the possibility that cholinergic synaptic transmission may occur from horizontal cell processes to photoreceptors in the outer plexiform layer of the turtle retina.

Retinal ganglion cells with NADPH-diaphorase activity in the chick form a regular mosaic with a strong dorsoventral asymmetry that can be modelled by a minimal spacing rule

We have identified a class of retinal ganglion cells in the chick retina that can be labelled by NADPH-diaphorase histochemistry. These cells have a remarkable topographic distribution, being restricted to the dorsal hemiretina, and form a highly regular mosaic, as revealed by the analysis of nearest neighbour distribution and Delaunay triangulation. Autocorrelation analysis of the mosaic of NADPH-diaphorase-positive retinal ganglion cells shows that the mosaic spatial organization could be generated with the single constraint that two elements cannot be closer than a given minimal distance (d(min)), which we confirmed by computer simulations. In contrast with what has been observed in other mosaics, here d(min) varies with cell density. However, the observed variation of the exclusion area is consistent with an original assembly of the mosaic with a constant d(min) (as is the case in other mosaics), followed by differential expansion of the retina during development.