Using immunohistochemical techniques, we determined localization of the zones of positivity with respect to cystathionine-β-synthase (CBS, the enzyme mediating production of hydrogen sulfide, H 2 S), GABA, and parvalbumin (PA) in the brain of the masu salmon (Oncorhynchus masоu). The corresponding positivity was found in neurons of the ventral spinal column (VSC), medial reticular formation (RF) of the myelencephalon, fibers and cells of the cerebellum, in the mesencephalic tegmentum, optic tectum, periventricular diencephalon, hypothalamus, and telencephalon. The heterogeneity of immunopositive CBS-, GABA-, and PA-containing neuronal subpopulations in all cerebral regions of the masu salmon is, obviously, related to the fact that the above-mentioned groups belong to different neurochemical and electrophysiological systems. In caudal magnocellular cerebral systems (dorsomedial nuclei of the tegmentum, medullary parts of the medial RF, and also in the VSC), we found high levels of colocalization of CBS, GABA, and PA. We hypothesize that hydrogen sulfide, which is produced by CBS-containing cells, can play a role of the modulator of GABAergic neurotransmission, while the presence of PA in neuronal elements is indicative of a high level of energy metabolism and/or high electrical activity. In the periventricular parts of the diencephalon of the masu salmon, we found cells with high levels of CBS-, GABA-, and PA-positivity. It seems possible that these cells are involved in the processes of growth and physiological regeneration of neurons in diencephalic cerebral structures of fishes.

Using techniques of immunoperoxidase staining of proliferating cell nuclear antigen (PCNA) and TUNEL labeling of fragmented DNA, we studied sites of proliferation and apoptosis in the myelencephalon, cerebellum, tectum opticum, thalamus, and hypothalamus of the Amur sturgeon (Acipenser schrenckii). We found that the processes of proliferation and apoptosis are maintained in the brain of 3-year-old sturgeon individuals; the ratio of these processes in different cerebral regions varied significantly. The maximum intensity of proliferative activity was found in the periventricular zone of the myelencephalon (proliferation index, on average, 21.0 ± 1.3%). This fact allows us to consider this cerebral region a most important zone were adult neurogenesis occurs in the sturgeon. In the medial reticular formation, dorsal thalamic nuclei, inner fibrous layer of the tectum, and lateral hypothalamus, the maximum numbers of apoptotic elements were found. Therefore, these zones in the brain of the sturgeon correspond, apparently, to the regions where postmitotic neuroblasts are localized. In sensory centers (tectum and nuclei of the V, VII, and X nerves), significantly varying ratios of intensities of proliferation and apoptosis were found; this is indicative of dissimilar rates of growth and differentiation in visual and chemosensory centers of the sturgeon brain. The high proliferative activity in sensory and motor cerebral centers of the sturgeon allows us to hypothesize that a neotenic pattern is preserved in these CNS regions of adult sturgeons over a long period after the embryogenesis has been completed.

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Using immunohistochemical labeling of the cells containing neuronal NO synthase (nNOS), tyrosine hydroxylase (TH), GABA, and parvalbumin (PA), as well as histochemical marking of choline acetyltransferase-containing neurons, we examined the neurochemical organization of the glomerular nuclei and preglomerular complex in the brain of the masu salmon (Oncorhynchus masou). Injections of the carbocyanine dye DiI allowed us to examine projections of neurons of the preglomerular and mammillary nuclei in the salmon brain. We showed that cholinergic, GABA-, PA-, TH-, and nNOS-immunopositive neurons belonging to different morphological types are present in the glomerular and medial preglomerular nuclei. The analysis of correlations between morphometric characteristics of the cells belonging to different neurochemical types and densitometric estimates of amounts of neurochemical agents present in these cells allowed us to hypothesize that there are close morphofunctional interrelations in cell populations possessing different neurochemical and morphometric characteristics. These interrelations of the cells belonging to different chemotypes are, probably, realized as mediatory/modulatory ones. The presence of a great number of small slightly differentiated cells in the preglomerular and glomerular nuclei allows us to suppose that the growth of the greatest sensory center of the salmon brain is provided by neuroblasts that migrate from the proliferative zones in the course of postembryonal neurogenesis. It is also hypothesized that NO, TH, and GABA are involved in paracrine control of the postnatal morphogenesis of the salmon preglomerular complex. The data obtained by hodological analysis indicate that the nuclei of the preglomerular complex obtain afferent projections from the dorsomedial and ventroventral telencephalic regions, preoptic nucleus, periventricular layer of the tectum, and posterior central thalamic nucleus. Our study demonstrated the existence of reciprocal functional connections between the preglomerular complex (most important diencephalic center for transmission of sensory information) and dorsomedial and ventral regions of the telencephalon in the masu salmon.