The Involvement and Functional Role of the Fish Nonapeptidergic Preoptico-Hypophysial Neurosecretory System in Spawning Migrations.

The involvement of the preoptico- or hypothalamo-hypophysial neurosecretory system (HHNS) in the initiation of migration processes in fish has been discovered, based on the results of ecologo-histophysiological studies using light microscopy, immunohistochemistry, electron microscopy and morphometry, thus confirming the hypothesis proposed by N.L. Gerbilsky and A.L. Polenov about the nature of the migratory impulse in fish. It has been demonstrated for the first time that in genetically distant but ecologically similar anadromous migratory fish species with different breeding seasons (spring-spawning Russian sturgeon, stellate sturgeon, and fall-spawning pink salmon), the synthesis of neurosecretory products is activated in neurosecretory cells at the beginning of the migration process, followed by their transport and accumulation in the neurohypophysis. At the same time, neurosecretory products have been shown to be secreted into the cerebrospinal liquid of the 3rd ventricle. Thus, the HHNS has a complex synchronous effect, which includes: (1) the neurotropic action of nonapeptide neurohormones on behavior centers of the central nervous system (CNS), causing a dominant state of CNS excitation originally called the "migratory impulse", (2) osmoregulatory imbalance during the marine "fattening" period, and (3) the cessation of the known antigonadotropic action of nonapeptide neurohormones, thus promoting the switching of the organism to an energy-intensive type of energy metabolism. A comparative analysis of our own and literature data confirms the communality of this metabolic mechanism in fish, thus allowing it to be considered as the most important phylogenetic adaptation (aromorphosis) aimed at achieving biological progress of species. A leading role in the subsequent navigational mechanisms of homing, widely covered in the world literature, is played by the luliberinergic centers of the hypothalamus. The ecological diversity of these mechanisms at the population level and the presence of straying in fish allow considering them as microevolutionary specializations. Further analysis of HHNS functions during fish migrations and spawning will help establish the basic principle of integration of these processes at the given ontogenetic stage, which is of prime importance for the existence of species. [ABSTRACT FROM AUTHOR]