A complete study of neuroendocrine regulation of fish reproduction 6. Functional role of the hypothalamo-hypophysial neurosecretory system in the integration of fish reproduction

The participation of the hypothalamic-pituitary neurosecretory system (NS) in spawning was shown in the previous article in the form of activation synthesis and excretion of nonapeptide neurohormones (NPh) at its beginning, and the subsequent functional activity decrease after its completion (in the studied spring-, autumn- and winter-spawning fish species gg.: Acipenser, Oncorhynchus, Lota), regardless of the spawning season. The purpose of this study is to clarify the NS functional role in the reproduction process of valuable commercial fish species with various biological features of spawning. It has been established that at the beginning of spawning and up to its completion, the NS initiates spawning behavior, affecting the central nervous system by the transventricular route and participates in the acquisition and preservation of spawning coloration, affecting the melanotropocytes of the adenohypophysis by the tranadenohypophyseal route. At the time of ovulation and spermiation, nonapeptide neurohormones (NPh) of the NS stimulate smooth muscle contractions. During spawning period, the NPh participation in maintaining the osmotic body balance is important, since the progressive muscle watering occurs in salmon at the process of river migration and spawning. The most important synchronous effect, caused by the maximum increase of the NS viscerotropic activity during spawning period, is the NPh antigonadotropic action, which is carried out by their inhibiting direct influence to all levels of the hypothalamic-hypophysial-gonadal axis of neuro-endocrine relationships. Spawning is concluded by the mass excretion of NPh into neurohypophysial vessels of the general blood circulation, that reflects the NS participation in the body protective-adaptive reactions to the natural physiological stress that occurs during the reproduction period in polycyclic fish. In monocyclic species, immediately after spawning, there is a blockade of the NPh excretion from the posterior neurohypophysis, corresponding to the extreme CNS inhibition under distress. The functional NS role in fish reproduction is to initiate energy-consuming reproductive migratory and spawning behavior processes and to complete spawning by suppressing the hyperactivity of the target glands, which ensures the transition of the body to energy-saving plastic metabolism. Constructive schemes of NS participation in the spawning process are presented.