Effect of experimental 3,5,3'-triiodothyronine hyperthyroidism on thyroid hormone deiodination in brain regions and liver of rainbow trout,Oncorhynchus mykiss

Abstract

We studied the effect of 3,5,3'-triiodothyronine (T3) hyperthyroidism, induced by 12 ppm T3in food for 10 days, on the low-Kmactivities of thyroxine (T4) outer-ring deiodination (ORD) to form T3, T4inner-ring deiodination (IRD) to form 3,3',5'-triiodothyronine (reverse T3(rT3)), T3ORD to form 3,5-diiodothyronine (3,5-T2), and T3IRD to form 3,3'-diiodothyronine (3,3'-T2) in six brain regions and in liver of immature rainbow trout (Oncorhynchus mykiss) at 12°C. Throughout the brain, T4ORD activity of control trout was uniformly low and T3ORD activity was negligible. T4IRD and T3IRD activities were about 5-fold and 50-fold greater, respectively, than T4ORD activity and were higher in the optic lobes, hypothalamus, and telencephalon/olfactory bulbs than in the medulla or cerebellum. T3treatment doubled the plasma T3level with no change in plasma T4level and reduced T4ORD and T4IRD activities in all brain regions but did not alter T3IRD activity or the negligible T3ORD activity. Relative to controls, T3treatment reduced liver T4ORD activity 6-fold, increased T4IRD activity 8-fold, and increased T3IRD activity 12-fold. We conclude that (i) there are regional differences in trout brain T4IRD and T3IRD activities but not in T4ORD activity, indicating spatial variation in brain T4and T3catabolism, (ii) in response to a mild T3challenge the brain deiodination pathways do not undergo the same autoregulatory adjustments as those in liver, and (iii) a T3challenge reduces brain T4IRD activity with no change in T3IRD activity, which suggests that the two IRDs may be controlled by separate deiodinases.