Central administration of growth hormone-releasing hormone and corticotropin-releasing hormone stimulate downstream movement and thyroxine secretion in fall-smolting coho salmon (Oncorhynchus kisutch).

摘要

The ultimate signal triggering downstream migration in anadromous salmonids is unknown. A plasma surge of T(4) (T(4) surge) occurs during downstream migration in salmonids; however, the causal relationship between migratory behavior and the T(4) surge is not well known. We first examined the progression of smolt indicators (skin silvering, condition factor (CF), gill Na(+), K(+)-ATPase (NKA) activity and plasma T(4) levels) in underyearling, fall-smolting coho salmon (Oncorhynchus kisutch) from August to December. In November, the fish showed the characteristics of fully developed smolts, i.e. the skin completely covered with silvery scales, CF at a nadir, and peak NKA activity and plasma T(4) levels. Based on these results, we examined the effects of four neuropeptides, thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone (CRH), growth hormone-releasing hormone (GHRH), and gonadotropin-releasing hormone (GnRH), on the downstream movement (negative rheotaxis) and T(4) surge in fully smoltified underyearling coho salmon. The experiment was run in circular-shaped channel tanks and the neuropeptide treatment was performed as intracerebroventricular (ICV) injections. ICV injection of GHRH and CRH stimulated both downstream movement and plasma T(4) level. TRH injection stimulated plasma T(4) level but suppressed downstream movement. GnRH injection had no effect. It is hypothesized that GHRH and CRH play key roles in triggering downstream migration of anadromous salmonids, and that the accompanying T(4) surge is a consequence of the neuroendocrine processes that trigger migration.