Involvement of Gonadotropin-Releasing Hormone in Thyroxine Release in Three different forms of Teleost Fish: Barfin Founder, Masu Salmon and Goldfish

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Effects of gonadotropin-releasing hormone (GnRH) on thyroxine (T4) release in vivo and in vitro were studied in barfin flounder Verasper moseri, masu salmon Oncorhynchus masou and goldfish Carassius auratus. Seabream GnRH (sbGnRH) at a dose of 200 ng/50 g body weight (BW) significantly increased plasma T4 levels 1 h after the in vivo injection in the barfin flounder, but thereafter the levels normalized. Salmon GnRH (sGnRH) significantly increased plasma T4 levels l h after the injection with a significant return to initial levels in male masu salmon and male goldfish. In contrast, sGnRH and cGnRH-II in barfin flounder, and cGnRH-II in male masu salmon and male goldfish were not effective in stimulating T4 release. To clarify direct involvement of GnRH in T4 release, dissected lower jaw including scattered thyroid follicles was incubated with sbGnRH (1 μg/well) in barfin flounder, and with two doses (0.1 and 1 μg/well) of sGnRH in masu salmon and goldfish in vitro. T4 concentrations of control were stable during 24 h. Incubation of lower jaw with high dose (1 μg/well) of GnRH significantly (P<0.05) increased T4 concentrations of incubation medium at 1 h in all experimental fishes. These results indicate that direct stimulation of T4 secretion by GnRH occurs widely in teleost fish.

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