Effect of estradiol-17β on pituitary-thyroidal axis of Japanese eel (Anguilla japonica)

The effect of estradiol-17β (E₂)treatment on the pituitary-thyroidal axis has been investigated in female Japanese eels in seawater. The eels were injected with 1 mg/kg body weight (BW)weekly for two to eight weeks; another group was treated with 5 mg/kg BW weekly for eight weeks and sacrificed three days after the last injection. In the pituitary, using RT-PCR system, the thyroid stimulating hormone β subunit mRNA (TSHβ mRNA) level is slowly increased in the low-dose E₂ treatment group. A significant increase in values occurred after eight weeks E₂ injections. The histological structure of the thyroid follicles suggests a hypo-activity as shown by a reduced epithelial cell height in eels injected with E₂ after six and eight times, and eventually reached a minimum level after eight weeks. Using radioimmunoassay of thyroxine (T₄), triiodothyronine (T₃) in the serum, a slowly decrease of both hormones is demonstrated. On the other hand, the TSHβ mRNA level is significantly increased from initial in the high-dose E₂ treatment group. The histological structure of the thyroid follicles suggests a hypoactivity, and a sharp decrease of the both hormones is also indicated. These data suggest that the expression of TSHβ mRNA level is increased and thyroid hormone release is depressed by E₂, the effects depend upon E₂ injection period and dose. Furthermore, E₂ could not directly influence the TSHβ mRNA expression and, TSH transcription might be controlled by a negative feedback of thyroid hormone.