The effects of testosterone on upstream migratory behavior in masu salmon, Oncorhynchus masou

Evaluation of seasonal and daily changes of plasma thyroxine and cortisol levels in wild masu salmon Oncorhynchus masou, sampled by a Japanese fishing method

A new fish sampling method was developed using a Japanese bait fishing rod (8-9 m carbon rod and a nylon line with a small fine wire single hook), which is considered to catch wild salmonid juveniles with low sampling stress. Using this method, seasonal and daily changes of plasma thyroxine (T4 ) and cortisol levels were examined in wild parr, pre-smolts and smolts of masu salmon Oncorhynchus masou in contiguous locations in a coastal river (Kesen River; 44 km) in northern Honshu Island, Japan, overlapping the period of smoltification and seaward migration from August to March. Plasma T4 and cortisol were low in 0+ and 1+ year parr caught in August and September. In March, some yearling (1+ year) fish, which were judged as pre-smolts, and smolts appeared mainly in mid and lower reaches, while parr (0+ and 1+ year parr) continued to appear in the upper and mid reaches. In March, 1+ year pre-smolts and smolts showed high plasma T4 levels while the levels of 1+ year parr were low. During March 2008-2010, plasma T4 levels of 1+ year pre-smolts and smolts had high levels from early to mid-March, whereas plasma cortisol levels of 1+ year smolts were low in early March and increased towards mid-March. Based on these data, plasma cortisol increases probably occur following the increases of plasma T4 levels to lead the 1+ year O. masou to the completion of smoltification and initiation of seaward migration.

Physiological mechanisms of imprinting and homing migration in Pacific salmon Oncorhynchus spp

After several years of feeding at sea, salmonids have an amazing ability to migrate long distances from the open ocean to their natal stream to spawn. Three different research approaches from behavioural to molecular biological studies have been used to elucidate the physiological mechanisms underpinning salmonid imprinting and homing migration. The study was based on four anadromous Pacific salmon Oncorhynchus spp., pink salmon Oncorhynchus gorbuscha, chum salmon Oncorhynchus keta, sockeye salmon Oncorhynchus nerka and masu salmon Oncorhynchus masou, migrating from the North Pacific Ocean to the coast of Hokkaido, Japan, as well as lacustrine O. nerka and O. masou in Lake Toya, Hokkaido, where the lake serves as the model oceanic system. Behavioural studies using biotelemetry techniques showed swimming profiles from the Bering Sea to the coast of Hokkaido in O. keta as well as homing behaviours of lacustrine O. nerka and O. masou in Lake Toya. Endocrinological studies on hormone profiles in the brain-pituitary-gonad axis of O. keta, and lacustrine O. nerka identified the hormonal changes during homing migration. Neurophysiological studies revealed crucial roles of olfactory functions on imprinting and homing during downstream and upstream migration, respectively. These findings are discussed in relation to the physiological mechanisms of imprinting and homing migration in anadromous and lacustrine salmonids.

Sex steroids and their receptors in lampreys

The use of steroids and their receptors as ligand-gated transcription factors is thought to be an important step in vertebrate evolution. The lamprey is the earliest-evolving vertebrate to date in which sex steroids and their receptors have been demonstrated to have hormonal roles similar to those found in jawed vertebrates. Sex steroids and their receptors have been examined in several lamprey species, and the majority of studies have focused on the sea lamprey, Petromyzon marinus. While classical steroids appear to be present in lampreys, their function, concentrations, and synthesis have not been determined conclusively. The only classical steroid that is thought to act as a hormone in both males and females is estradiol. Recent research has established that lampreys produce and circulate 15alpha-hydroxylated steroids, and that these steroids respond to upstream stimulation within the hypothalamic-pituitary-gonadal axis. In particular, 15alpha-hydroxyprogesterone is highly sensitive and responds in great magnitude to stimulation, and is likely a hormone. Lampreys also appear to use androstenedione, a precursor to vertebrate androgens, as their main androgen, and a receptor for androstenedione has recently been described. Non-classical steroids are prevalent in many aquatic vertebrates, and the non-classical steroids found in the sea lamprey may represent an evolutionary artifact, or alternatively may be a way to avoid endocrine disruption when ingesting the body fluids of host fish. The lamprey will continue to be an interesting model for examining the evolution of steroid hormones, steroid receptors, and steroid function.

Year-to-year differences in plasma levels of steroid hormones in pre-spawning chum salmon

Changes in plasma levels of steroid hormones in pre-spawning chum salmon (Oncorhynchus keta) were examined for 6 years in association with sexual maturation. Fish were sampled along their homing pathway from the coastal sea to the spawning ground from 1995 to 2000. Plasma levels of testosterone (T), 11-ketotestosterone (11KT), estradiol-17beta (E2), 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP), and cortisol were determined by enzyme immunoassays. Sexual maturity was comprehensively estimated by gonadosomatic indices, histology of gonads, nuptial color, spermiation or ovulation ratio. Since the plasma levels of steroid hormones and sexual maturation differed from year to year, they were compared with year-to-year variation of sea surface temperature (SST) of coastal sea to study influence of oceanographic environment on these physiological data. The SST of the migratory route varied among the years, so that we classified the 6 years into cool, intermediate, and warm years. Concerning maturity, the males that returned to the natal hatchery in the warm years were sexually more advanced than those in the cool years. Furthermore, histological data suggested that final oocyte maturation occurred before arrival at the hatchery in one of the warm years, i.e., 1999, while it occurred at the hatchery in one of the intermediate years, i.e., 2000. In the males, T and 11KT levels increased significantly on midway of the homing route in the warm years, whereas they did not show any noticeable changes in the cool years. Furthermore, the levels of T and 11KT on midway of the homing route in the warm years, i.e., 1998 and 1999, were significantly higher than those in one of the cool years, i.e., 1995, in both sexes. In the females, the levels of E2 decreased during upstream migration. Conversely, those of DHP considerably elevated at spawning ground in all years examined. The levels of cortisol were different from year to year regardless of the SST. The present results showed that there were year-to-year differences in plasma levels of steroid hormones and maturity, and some of them may be influenced by the year-to-year variation of SST.