Kobayashi Award 2024: Cyclic regulation of the reproductive neuroendocrine system in fish: Using migratory fish as a...

Migration involves the coordinated regulation of various physiological functions and behaviors in order to adapt to complex environmental factors in the wild. Migratory fish are valuable models for studying the cyclic regulation of the reproductive neuroendocrine system due to their distinct and tightly timed annual and seasonal reproductive cycles synchronized with changing environmental factors. This review provides an overview of the cyclic regulation of the reproductive neuroendocrine system, focusing on GnRH, kisspeptin and GnIH, in two migratory fish: Pacific salmon, which undertake long-distance spawning migration in the North Pacific Ocean and the grass puffer, which repeatedly migrates to spawn in a coastal area, synchronizing with lunar phases and tidal cycles. Our studies on Pacific salmon revealed that the timeline of sexual maturation is likely common among Pacific salmon, regardless of species or location. During the spawning migration, GnRH3 plays a central role in gonadal maturation and migratory behavior in conjunction with IGF-I and E2. Its action on GTH secretion varies significantly depending on the stage of sexual maturation and the season. In the grass puffer, two different biological clocks, a circatidal clock and a circadian clock, may produce semilunar oscillations, and melatonin may play a role in synchronizing these clocks and the reproductive neuroendocrine system with the lunar phase. Our studies of these two migratory fish species provide valuable insights into how wild animals regulate their reproductive functions in response to periodic changes in diverse environments.