The functional role of hypothalamo- hypophysial neurosecretory system in fish migrations and spawning

The results of the ecological-histophysiological study of the hypothalamo-hypophysial neurosecretory system (HHNS) by use of light- and electron-microscopical morphometric methods and comparative analysis of the results are presented. At the beginning of migrations a state of “HHNS mobilization” has been established. It is expressed in the form of the strong synthesis activation of neurosecretary products in neurosecretory cells (NSC) and their transport in dendrites and axons to neurohypophysis (NH), where, their mass accumulation, however, occurs. This completion of moderate “normal” level of excretion of nonapeptide neurohormones (NP-NH) into the bloodstream disrupts the body’s water-salt homeostasis. The synchronized transventricular direction of the NP-NH excretion from dendrites and axons into the brain liquor of III ventricle provides their neurotropic effect in the behavioral centers of Central Nervous System (CNS). As a result, HHNS has a decisive double synchronous effect, which disrupts the long-adapted “pasture” type of osmoregulation (hyper- or hypoosmotic), which is probably the main physiological stimulus of habitat change and, at the same time, it includes the behavioral centers of the CNS, which causes a dominant state, originally defined as a “migration impulse.”The interactions of nonapeptide- and luliberinergic centers of the hypothalamus in the navigational mechanisms of homing and imprinting are discussed. At the beginning of spawning, regardless of its season a strong activation of HHNS, followed by a decrease in its functional activity, was discovered in fish of continuous spawning. The detected two-phase reaction of the HHNS seems to be a reflection of its participation in the body’s protective and adaptive responses to physiological stress. The key role of the HHNS in integrating migration and spawning processes is discussed.