Physiological Mechanisms of Imprinting and Homing Migration In Salmon

 Three different research approaches from behavioral to molecular biological studies have been applied to clarify physiological mechanisms of salmon imprinting and homing migration using four anadromous Pacific salmon (pink, chum, sockeye, and masu salmon) migrating from the North Pacific Ocean to Hokkaido, as well as lacustrine sockeye and masu salmon in Lake Toya, Hokkaido, where the lake serves as the model oceanic system. The first was behavioral approach by means of biotelemetry techniques. We investigated swimming profiles of maturing chum salmon migrating from the Bering Sea to the eastern Hokkaido using a micro-data logger with a propeller that can measure swimming speed, swimming depth, and ambient temperature of fish, and obtained swimming profiles over a period of 67 days in which fish traveled up to 2,763 km. We also tracked homing migration of sensory function-disturbed lacustrine sockeye and masu salmon in Lake Toya, and found that both visual and olfactory cues were important for these lacustrine salmon. The second approach was endocrinological research on hormone profiles in the brain-pituitary-gonadal axis of chum salmon migrating from the Bering Sea to the spawning ground in Hokkaido. Levels of two types of gonadotropin-releasing hormone (sGnRH and cIIGnRH), two types of gonadotropin (LH and FSH) in the pituitary, and gonadal steroid hormones were measured by specific time-resolved fluoroimmunoassay.  We also carried out in vitro brain slice experiment to clarify possible roles of N-Methyl-D-Aspartate (NMDA) receptor on GnRH functions. The third approach was molecular and physiological research on olfactory functions that play a crucial role in imprint and discrimination of the natal stream odorant during downstream and upstream migration, respectively. We cloned and characterized a salmon olfactory system imprinting-related gene and salmon odorant receptor genes. We also compared whether dissolved free amino acid (DFAA) mixture of their natal stream have attractive effects on upstream selective movement among four Pacific salmon. Moreover, to investigate odor information processing of the natal stream, we applied blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to investigate the response to the natal stream water in the olfactory bulb and telencephalon of lacustrine sockeye salmon. These results demonstrate the navigation ability in open water, the hormone profiles in the brain-pituitary-gonadal axis, and the olfactory functions related with imprinting and discriminating ability of natal stream odorants in salmon. These recent findings are discussed in relation to physiological mechanisms of the amazing imprinting and homing migration in anadromous and lacustrine salmon.