Expression of hormone genes and osmoregulation in homing chum salmon: A minireview

Abstract

Pacific salmon migrate from ocean through the natal river for spawning. Information on expression of genes encoding osmoregulatory hormones and migratory behavior is important for understanding of molecular events that underlie osmoregulation of homing salmon. In the present article, regulation of gene expression for osmoregulatory hormones in pre-spawning salmon was briefly reviewed with special reference to neurohypophysial hormone, vasotocin (VT), and pituitary hormones, growth hormone (GH) and prolactin (PRL).Thereafter, we introduced recent data on migratory behavior from SW to FW environment.In pre-spawning chum salmon, the hypothalamic VT mRNA levels increased in the males, while decreased in the females with loss of salinity tolerance when they were kept in SW.The amounts of GH mRNA in the pituitary decreased during ocean migration prior to entrance into FW. Hypo-osmotic stimulation by SW-to-FW transfer did not significantly affect the amount of PRL mRNA, but it was elevated in both SW and FW environments along with progress in final maturation. Behaviorally, homing chum salmon continued vertical movement between SW and FW layers in the mouth of the natal river for about 12 hours prior to upstream migration. Pre-spawning chum salmon in an aquarium, which allowed fish free access to SW and FW, showed that individuals with the lower plasma testosterone (T) and higher estradiol-17β (E2) levels spent longer time in FW when compared with the SW fish.Taken together, neuroendocrine mechanisms that underlie salt and water homeostasis and migratory behavior from SW to FW may be under the control of the hypothalamus-pituitary -gonadal axis in pre-spawning salmon.

Activity of the pituitary–gonadal axis is increased prior to the onset of spawning migration of chum salmon

“…The distance from the mouth of the Ishikari River to the Chitose Field Station is about 70 km. Analyses of data loggers attached to homing adults in the Ishikari Bay showed that fish spent a few weeks to reach the hatchery from the mouth of the Ishikari River (Kitahashi et al, 2000;Makino et al, 2007).…”

“…Elevated levels of sex steroid hormones have been found to be associated with the adaptation to FW in pre-spawning chum salmon (Makino et al, 2007). Makino et al found that the plasma levels of T and/or E2 were related to the amount of time spent in FW, when spontaneous behavior of pre-spawning chum salmon was monitored in an aquarium that allowed fish to migrate between separated SW and FW streams (Makino et al, 2007). An application of T, 11KT or E2 directly modulated prolactin (PRL) gene expression, a FW adaptation hormone (Hirano et al, 1987), in primary pituitary cells of pre-spawning and mature masu salmon .…”

“…Homing adults, which leave the Bering Sea in early summer, require two to three months to reach their natal rivers in Japan (Tanaka et al, 2005). Then, they adapt to FW and complete their final maturation during upstream migration in autumn (Makino et al, 2007).…”

Effects of Experimental Manipulations of Salinity and Maturation Status on the Physiological Condition and Mortality of Homing Adult Sockeye Salmon Held in a Laboratory

“…Stimulation of the hypothalmus-pituitary-inter renal axis produces an increase in plasma cortisol concentrations, along with changes in other compounds associated with FW entry (Clarke and Hirano 1995;McCormick 2001;Mancera and Fuentes 2006;Norris and Hobbs 2006;Makino et al 2007). Included among the physiological responses are a shift in the number, distribution, and function of gill chloride cells (Uchida et al 1997) and the downregulation of gill Na*,K^ ATPase activity (Shrimpton et al 2005).…”

Effect of hypo-osmotic environmental changes on the expression of gonadotropin-releasing hormone, its receptor, and gonadotropin hormone subunit mRNA in adult chum salmon (Oncorhynchus keta)

“…GTH plasma levels also increase during upstream migration in pre-spawning chum salmon (Ueda et al 1984;Kitahashi et al 1998), pink salmon (Dye et al 1986), and sockeye salmon (Truscott et al 1986), although it should be noted that these studies mainly detected LH because antiserum that distinguishes FSH from LH was not used. Makino et al (2007) reported that elevated levels of sex steroid hormones, such as E 2 , are associated with FW adaptation in pre-spawning chum salmon, and the application of sex steroid hormones has been shown to directly modulate the PRL gene during FW acclimation in adult chum salmon (Hirano et al 1987;Onuma et al 2005). Therefore, it is likely that the endocrine systems that govern salt and water homeostasis metabolism in homing salmon are regulated by PG axis activity.…”