Functional specificity of gonadotropin and thyrotropin in Fundulus heteroclitus

Gonadotropic hormones (GTHs) and thyrotropic hormones (TSHs) reportedly bear close evolutionary and structural relationships, and the thyroid appears to be active in reproduction in some fish species. We tested the sensitivity of the thyroid of Fundulus heteroclitus to glycoprotein hormones from mammalian and piscine sources. Six mammalian glycoprotein hormones, including four gonadotropins and two thyrotropins, produced dose-dependent elevations in serum thyroxin. A release of endogenous gonadotropins was elicited by injecting GnRH. This resulted in gonadal stimulation, with no alteration in circulating thyroxin levels and the rate of radioiodine uptake. We also treated fish with partially purified salmon gonadotropin (SG-G100). The gonadotropic actions of this extract were confirmed by steroid elevations, and again T4 and 125I uptake remained at resting levels. The lack of response of the thyroid gland to fish gonadotropins suggests that TSH receptors in Fundulus heteroclitus can differentiate between endogenous thyrotropin and gonadotropin(s), even though most heterologous glycoprotein hormones are thyrotropic.