A complete study of neuroendocrine regulation of fish reproduction. 5. Participation of the hypothalamo-hypophysial neurosecretory system in fish spawning

In the previous work, the starting mechanism of neuroendocrine regulation of fish reproduction was described in the form of the “Migration Impulse”, where for the first time in the world literature, a complex synchronous effect of the participation of the hypothalamic-hypophysial neurosecretory system (NS) in the initiation of fish reproduction was shown on the basis of ecological-histophysiological research method. The purpose of this study was to clarify the participation degree of NS in the reproduction process of valuable fish species (Acipenseridae and Salmonidae) with various biological features of spawning. The participation of NS in the spawning process was shown by histomorphological, immunohistochemical and electron-microscopic techniques using quantitative morphometry. Processes activation of nonapeptide neurohormones synthesis and excretion at the beginning and soon after spawning and subsequent decrease of their functional activity after its completion was revealed in each fish species, regardless of the spawning season (spring, autumn and winter spawning: Acipenser, Oncorhynchus, Lota). A similar reaction of NS can be traced in experiments under the stress effect of a hypertonic medium on adult Acipenseridae fish. The biphasic reaction of NS, corresponding to stress stages of anxiety (alarm reaction) and resistance, is considered as a reflection of its participation in ensuring the body protective and adaptive reactions to physiological stress that occurs during the reproduction period of polycyclic fish. In monocyclic species, immediately after spawning, there is a blockade in the excretion of neurohormones from the posterior neurohypophysis (NH), corresponding to the extreme inhibition of NS under stress. Thus, NS initiates spawning and completes it by participating in the body’s protective and adaptive reactions to natural physiological stress.