Ecologic-histophysiological overview of the involvement of the hypothalamo-hypophysial neurosecretory system in fish reproduction

Abstract

The participation of the hypothalamo-hypophysial neurosecretory system (HHNS) in fish reproduction was shown by histomorphological and electronic microscopical studies with the use of quantitative morphometry. The activation of HHNS at the beginning of spawning and the following decrease of its functional activity was revealed in all studied one-time spawning fish species independently of the spawning season (based on spring-, autumn- and winter-spawning genera: Acipenser, Oncorhynchus, and Lota respectively). The diphasic reaction of HHNS corresponding to stages of “an alarm and resistance to stress”, is considered to be the reflection of its participation in protective-adaptive reactions of an organism to a physiological stress. In monocyclic species, right after spawning, there becomes the blockade of neurohormone releasing function from neurohypophysis corresponding to supernatural inhibition of system at disstress. At the beginning of spawning nonapeptide neurohormones (NpNh) of HHNS initiate spawning behavior and the appearance of “mating attire” by exposure to the central nervous system, pituitary gland and complex visceral organs. Then they promote ovulation and spermiation by stimulating the contraction of the smooth muscles of gonad. By the end of reproduction, they participate in the implementation of the body’s adaptations, aimed at overcoming physiological stress-spawning. Maintaining the body’s metabolic equilibrium is ensured by the pronounced anti-gonadotropic NpNh effect by inhibiting the gonadoliberin secretion and stimulating at the same time its antagonist – adrenocorticotropin secretion, as well as their direct effect on endocrine and generative gonad’s functions. This effect is crucial for the normalization of the physiological body state after spawning, as it allows to radically affect the nature of metabolic processes, by “switching” them from generative to plastic metabolism. A constructive working scheme of neuroendocrine regulation fish reproduction – its initiation (stimulating neurohormonal effect) and completion (inhibitory effect) by the self-regulation principle is presented. The important HHNS functional role in the integration of fish reproduction and the intended mechanisms for its participation in spawning migrations are discussed.