Adult Neurogenesis of Teleost Fish Determines High Neuronal Plasticity and Regeneration

Studying the properties of neural stem progenitor cells (NSPCs) in a fish model will provide new information about the organization of neurogenic niches containing embryonic and adult neural stem cells, reflecting their development, origin cell lines and proliferative dynamics. Currently, the molecular signatures of these populations in homeostasis and repair in the vertebrate forebrain are being intensively studied. Outside the telencephalon, the regenerative plasticity of NSPCs and their biological significance have not yet been practically studied. The impressive capacity of juvenile salmon to regenerate brain suggests that most NSPCs are likely multipotent, as they are capable of replacing virtually all cell lineages lost during injury, including neuroepithelial cells, radial glia, oligodendrocytes, and neurons. However, the unique regenerative profile of individual cell phenotypes in the diverse niches of brain stem cells remains unclear. Various types of neuronal precursors, as previously shown, are contained in sufficient numbers in different parts of the brain in juvenile Pacific salmon. This review article aims to provide an update on NSPCs in the brain of common models of zebrafish and other fish species, including Pacific salmon, and the involvement of these cells in homeostatic brain growth as well as reparative processes during the postraumatic period. Additionally, new data are presented on the participation of astrocytic glia in the functioning of neural circuits and animal behavior. Thus, from a molecular aspect, zebrafish radial glia cells are seen to be similar to mammalian astrocytes, and can therefore also be referred to as astroglia. However, a question exists as to if zebrafish astroglia cells interact functionally with neurons, in a similar way to their mammalian counterparts. Future studies of this fish will complement those on rodents and provide important information about the cellular and physiological processes underlying astroglial function that modulate neural activity and behavior in animals.

[Gaseous intermediates in the brain of the salmon Oncorhynchus masou]

Distribution of nitroxidergic and H2S-producing neurons in the brain of the salmon Oncorhynchus masou was studied by methods of histochemical markering of NADPH-diaphorase and by immunohistochemical markering of the neuronal nitric oxide synthase and cystathionin beta-synthase (CBS). The established distribution of CBS and nNOS/NADPH-d of neurons and fibers in the salmon telencephalon, optic tectum, and cerebellum allows suggesting that the NO- and H2S-producing systems, represent individual, non-overlapping neuronal complexes performing specialized functions in the activity of local neuronal networks. In the brainstem part, the nNOS-ir and NADPH-d-positive neurons were detected in the composition of viscerosensor (V, VII, and IX-X) and visceromotor (III, IV, and VI) nuclei of craniocerebral nerves, octavolateral afferent complex, reticulospinal neurons, and medial reticular formation. CBS in the salmon medulla was revealed in neurons of the X nerve nucleus, reticulospinal neurons, and ventrolateral reticular formation. Distribution of NO-ergical and H2S-producing neurons in the salmon medulla nuclei indicates that NO in salmon is the predominant neuromodulator of medulla viscerosensory systems, while H2S seems to modulate only the descending motor systems. The results of the performed study allow suggesting that NI in the descending motor systems. The results of the performed study allow suggesting that NO in the salmon medulla periventricular area can act as a regulator of postnatal ontogenesis.

[Neurochemical markers of cells of the periventricular brain area in the masu salm on Oncorhynchus masou (Salmonidae)]

Localization of GABA, tyrosine hydroxylase (TH), NADPH-diaphorase, transcription factor Pax6, and the proliferative cell antigen (PCNA) in the periventricular area of the brain of the masu salmon Oncorhynchus masou of varying age groups was studied. The presence of heterogeneous cell populations with radially orientated outgrowths in the diencephalon, central gray matter of the dorsomedial tegmentum, medulla oblongata, and spinal cord of the masu salmon was shown. TH-, GABA-immunopositive, and NADPH-d-positive cells are located in the areas of PCNA-immunogenic proliferative zones. It is possible that these cells are descendants of the radial neuroglia and that they participate in the growth and physiological regeneration of diencephalic and medullar structures. In various age groups of the masu salmon, the marking of TH-, GABA, PCNA, and Pax6 reveals the neuromeric structure of the brain.

[Morpho-functional and hodological peculiarities of ascending dopaminergic system of Rhodeus sericeus (Cyprinidae)]

Using the methods of tyrosine hydroxylase (TH) immunohistochemistry combined with nerve fiber labeling with carbocyanin dye DiI (1,1',dioctadecyl-3,3;3',3'-tetramethylindocarbocyanine perchlorate), the distribution of catecholaminergic (CE) neurons and their ascending projections to the basal telencephalon were studied in adult Amur cypriniform fish Rhodeus sericeus. Highly specialized complex of CE neurons was demonstrated in diencephalon. Six populations of TH-immunopositive neurons were detected in the area of posterior tuberculum, two of them were double-labeled, indicating the presence of dopaminergic projections to the ventral portion of the telencephalon (striatum). In the posteriotuberal area, two populations of CE cells were identified (small round neurons and large pear-shaped cells) which probably could correspond to the populations of sensomotor and limbic cells in mammals.

[NADPH-diaphorase, nitric oxide synthase, and tyrosine hydroxylase in the diencephalon of the Rhodeus sericeus (Cyprynidae:Teleostei)]

The presence and distribution of nitric oxide synthase (NOS)-like neurons as well as tyrosine hydroxylase-immunoreactive (TH) neurons was studied in the diencephalon of the cypriniform teleost Rhodeus sericeus. The anatomical relationships between tyrosine hydroxylase (TH)- and nitric oxide synthase (NOS)-containing cells were visualized both by NOS-immunohistochemistry and NADPH-histochemistry. Immunohistochemical labeling and morphological studies were performed on the same sections. The results reported in this paper show that both a NOS and TH activity are present in the preoptic region, posterior tuberculum, paraventricular organ and hypothalamus of R. sericeus. Putative nitrergic neurons were identified in all major hypophysiotrophic nuclei of the R. sericeus brain using both NADPH-d histochemistry and nNOS immunohistochemistry. In the preoptic region, nitrergic neurons were found in both the parvocellular and the magnocellular nuclei. Within these nuclei, the distribution of NADPH-d reactivity was similar to that of nNOS immunoreactivity. However, we found no evidence of colocalization of NADPH-d and nNOS in consecutive sections. NOS- and TH-containing neurons were observed in all the nuclei under study (hypothalamus, posterior tuberculum, ventral thalamus) and telencephalon (preoptic region), although most neurons showing the coexistence of both substances were mainly located in the preoptic nucleus and hypothalamus, some labelled neurons were found in the posterior tuberculum. Most of the cerebrospinalliquor-contacting cells (LCNs) in diencephalic periventricular area of R. sericeus were TH-immunoreactive. Also, a large number ofnitrergic small LCNs distributed throughout the third ventricle were observed in these regions. The data obtained supports the existence of a nitrergic circumventricular system in teleost. LCNs in R. sericeus are thought to be involved in osmoregulatory functions as osmosensitive neurons. Due to their chemical properties, NO produced by these cells might play an important role in the maintenance and regulation of CSF homeostasis through the modulation of cerebral blood flow.

[Tyrosine hydroxylase in telencephalon and diencephalon of Rhodeus sericeus (Cyprinidae)]

Immunohistochemical labeling of tyrosine hydroxylase was used to demonstrate catecholaminergic neuronal populations in the telencephalon and diencephalonof adult cypryniform fish Rhodeus sericeus. Various immunoreactive cell populations have been found in the telencephalon (ventral, central and lateral nuclei of ventral telencephalic area). Immunoreactive cells and fibers were discovered in dorsal nucleus of ventral telencephalic area and supracomissural nucleus in the caudal part of the telencephalon. In the diencephalon, periventricular nuclei (preoptic, periventricular nucleus of posterior tuberculum and periventricular organ) contained considerable TH-ergic cells. High activity of tyrosine hydroxylase was revealed in the pretectal, ventro-medial, ventro-lateral and suprachiasmatic nuclei. Periventricular hypothalamic nuclei also displayed high activity of tyrosine hydroxylase. Pseudounipolar neurons prevailed in all TH-immunereactive structures of the telencephalon and diencephalon: numerous bipolar liquor-contacting cells were discovered in the periventricular nuclei. Large pear-shaped cells and bipolar TH-ergic cells were found in posterior tuberculum. These cells may be functionally related to the dopamine-acquiring system.

[Proliferative zones in the brain of the Amur sturgeon fry. Interactions with neuromeres and migration of secondary matrix zones]

Neurogenesis in the forebrain region was studied in the Amur sturgeon Acipenser schrenki fry using immunocytochemical marking of the proliferative nuclear antigen. The brain zones with high proliferative activities were located at the brain ventricle surface facing the periventricular cavity. In addition to the periventricular zone of primary proliferation, several secondary proliferative zones were found in the forebrain region of the Amur sturgeon.

[Nitric oxide-ergic organization of medullar cranial nuclei in teleost fishes]

Traditional neuromorphological and NADPH-diaphorase methods were used to study and compare the topography and localization of medullar nuclei NO-ergic neurons in four teleost species: Pholis nebelosis, Hexagramus octogrammus, Carassius carassius and Pholidapus dybowskii. The peculiarity of medullar nuclei organization in teleost fishes consists in the weak topological disconnection of sensor and motor components with pronounced heteromorphism in their structure and the presence of subpopulations in their composition. In the fishes studied, NADPH-diaphorase was found in the trygeminal, octavo-lateral, facial, glossopharingeal and vagal nuclei. The number of NO-ergic neurons in the trygeminal nucleus was marginal and made up 9-14 % of the total cell number. About 40-60 % of NO-ergic cells were found in the octavo-lateral, facial, glossopharingeal and vagal nuclei.

[Cytoarchitectonic and neurochemical properties of spinal cord in teleost fishes]

Traditional neuromorphological and NADPH-diaphorase methods were used to study the topography, morphology and neurochemical organization properties of spinal cord in teleosts fishes. The heterogeneous population of NO-producing motoneurons was revealed in the motor column of spinal cords from studied species. Dendrites of primary motoneurons formed rich plexus at the spinal segment periphery. This morphological pattern is determined by translational motion of the fishes in the water (trunk-tail movement), and has no connection with the origin of upper and lower extremities. The NO-producing capacity of spinal motoneurons shows their connection with premotor NO-ergic brain system, including over situated motor centers of reticular formation and descending projections of giant steam neurons (Mauthner and Muller cells). The NO-producing Rohon-Berd neurons were found in the dorso-medial part of spinal cord from studied fishes. These cells with the ascending propriospinal targets form spinal nociceptive system. Thus, the sense Rohon-Berd cells and most motor neurons of studied bony fishes are nitric oxide synthesizing ones. Spinal cord NO-synthesizing territories are situated in concordance with dorso-ventral histochemical gradient. Spinal cord interneurons of these fishes produce nitric oxide selectively. The quantity of NO-synthesizing reticular cells is determined by two main factors: the connection with the specialized neurochemical complexes, where NO is a specific neuromodulator, and individual properties of spinal cord structure directed by conditions of morphoadaptation.

[Distribution of cholineacetyltransferase histochemistry in isthmus and medulla of Onchorynchus masu. Tract-tracing observation on the ascending meso-pontine cholinergic system]

The distribution of cholinergic neurons was studies in the brain steam, medulla and rostral spinal cord of the salmon Onchorynchus masu using histochemical choline acetyltransferase (ChAT) detection. Cholinergic neurons were observed in the isthmus, cranial nerve motor nuclei and spinal cord. In order to characterize several cholinergic nuclei observed in the isthmus of O. masu, their projections were studied by application of 1,1'-dioctadecyl-3,3,3',3,'-tetramethylindocarbocyanine perchlorate (DiI) to selected structures of the brain. The secondary gustatory nucleus projected mainly to the lateral hypothalamic lobes, whereas the nucleus isthmi projected to the optic tectum and parvocellular superficial pretectal nucleus, as it was earlier described for the other teleost group. In addition, the other isthmic cholinergic nuclei in O. masu may be homologous to the meso-pontine system of mammals. We conclude that the cholinergic systems of teleosts show many primitive features that have been presented during evolution, together with exclusive to the group characteristics.

[Structure and neurochemistry of the raphe nuclei in teleosts]

NO is an important modulator of the brain serotoninergic function as it is co-localized with 5-hydroxytryptamine in the neurons of raphe nuclei in the brain of all the higher vertebrates. In the brain of fishes this property of NO is poorly studied. The purpose of this work was to study the neuromorphological peculiarities and distribution of NO-ergic neurons in raphe nuclei of Teleost species--carps and perch-like fishes. Using standard neuromorphological methods and the histochemical reaction demonstrating NADPH-diaphorase activity, the comparative analysis of NO-ergic neurons was performed and their localization was studied in the raphe complex of five teleost species. Within the raphe complex in fishes two main nuclei--nucleus raphe superior and nucleus raphe inferior were demonstrated. NADPH-diaphorase activity was observed in practically all the neurons of raphe niclei in species examined. Some intergroup morpho-adaptational neurochemical differences were found in the of the ventro-medial reticular formation adjacent to the raphe complex.

[Morphological and functional features of the organization of perineuronal satellites of the optic nerve of Pholidapus dybowskii]

Glial cells in the optic nerve of Pholidapus dybowskii were observed by the Golgi silver impregnation method. Subtypes of protoplasmatic astroglia (free strongly ramified glia), perivascular macroglia (connected with vessels, ramified glia), and oligodendroglia were selected in concordance with the position of cell plexus. By correlative analysis of cell morphological characteristics, obtained by the Leontovich method of cell measuring, some cell features were discovered. The features positively correlated with the capacity of cell branch territory. Correlation coefficients of these features varied from 0.40 to 0.98. Groups of features with a high correlation coefficient were assumed as a basis for recognition of the detailed glial subtypes. Valid differences (P = 0.95) between the detailed glial subtypes were found. The main feature has a reliable difference equal to P = 0.99. Group of constant features were discovered, which define the overall performance of glial cells. Among glial cells, modification of capacities of cell branch territory was measured.

[Morphological organization of large Golgi neurons in the Pholidapus dybowskii cerebellum]

Type II large Golgi neurons (LGN) were studied in the cerebellum of Pholidapus dybowskii by silver impregnation technique. In the cerebellum LGN were revealed in three areas: the boundary zone between molecular and granular layers, lateral granular eminentia and granular layer. In the boundary zone, a horizontal orientation (along Purkinje cells layer) of LGN dendrites prevailed over a vertical one (in the direction from the granular layer to molecular one). In the granular eminentia, LGN dendrites exhibited a radial pattern without a pronounced asymmetry. In this zone, LGN were characterized with a higher number of primary dendrites compared to those of other zones. No LGN possessing more than one axon were found. In LGN of the boundary zone, axons oriented from either axonal hillock or proximal portions of large dendrites, the latter mode only occurring in LGN of the other ones.