Gonadotropin-Releasing Hormone Stimulates Thyroid Activity in a Freshwater Murrel, Channa gachua (Ham.), and Carps, Catla catla (Ham.) and Cirrhinus mrigala (Ham.)

Abstract

Injections of mammalian GnRH (mGnRH), salmon GnRH (sGnRH), and a homologous murrel, Channa punctatus, GnRH (cGnRH) to a murrel, Channa gachua, and a carp, Catla catla, at a dose of 1 μg/250 g body wt significantly increased plasma thyroxine (T4) levels above control. Piscine GnRHs (sGnRH and cGnRH) had significantly greater stimulatory effects compared with mGnRH. To observe whether this stimulatory effect by GnRHs is direct or indirect, thyroid follicles were isolated from hypobranchial muscles of freshwater murrel, C. gachua and incubated (1×106 follicles/well) in vitro at 30°C for 2 h without hormone and for 3 h with hormones. Addition of these three GnRHs separately at a concentration of 1 μg/well stimulated T4 secretion; sGnRH and cGnRH caused greater secretion of T4 into the medium compared with mGnRH. Specificity of GnRH action in vitro was assessed by using anti-GnRH antibody which significantly (P<0.01) inhibited GnRH-augmented T4 secretion. To gain further insight, 125I uptake by thyroid follicles and formation of [125I]T4 from this radioiodine pool was monitored in the presence or absence of sGnRH. sGnRH greatly augmented 125I uptake by the follicles which resulted a fourfold increase in [125I]T4 formation out of this pool of 125I compared with the control. The results indicate GnRH stimulation of thyroid hormone formation and release in these teleosts suggesting a possible different mode of regulation of thyroid hormone secretion in teleosts.