WIENS, S.C.*; EALES, J.G.: Effects of 17-β-estradiol on thyroid hormone deiodination in liver and brain of rainbow trout (Oncorhynchus mykiss) at different reproductive states

Reproductive hormones affect thyroid system function in rainbow trout. 17-β-estradiol (E2) effects on thyroid hormone (TH) deiodination in sexually differentiated rainbow trout were investigated by measuring liver and brain microsomal deiodination activity in trout at different reproductive states. Trout (in order of decreasing sexual maturity: Group I=800-2000g; Group II=400-600g; Group III=200-400g) were injected intraperitoneally with E2 in peanut oil (0.5mg/100g body weight) or peanut oil alone on days 1 and 3, and sacrificed on day 7. Deiodination activity was measured in liver and brain for three deiodination pathways: T4-outer-ring deiodination (T4-ORD), producing 3,5,3′-triiodothyronine (T3, the biologically active TH) from thyroxine (T4); T4-inner-ring deiodination, producing inactive 3,3′,5′-triiodothyronine; T3-inner-ring-deiodination (T3-IRD), producing inactive 3,3′-diiodothyronine. Liver T4-ORD was significantly lower in E2-treated trout of Groups I and II. Brain deiodination was not affected, except for T3-IRD in Group II. Group II control males had lower brain T3-IRD than females, but E2-treated males had higher T3-IRD than controls, similar to control female levels. These data confirm previous results on E2 inhibition of liver T4-ORD in sexually undifferentiated, juvenile rainbow trout, and additionally demonstrate no E2 effect on other hepatic deiodination pathways. The E2-induction of T3-IRD in brain of males to female control levels in less sexually mature trout suggests some E2 role in regulating brain T3-IRD activity at specific reproductive states. Group III analysis may support this role of E2. E2 effects on TH deiodination may reflect potential effects of environmental estrogens on wild fish.