Parr-smolt transformation in Atlantic salmon: thyroid hormone deiodinationin liver and brain and endocrine correlates of change in rheotactic behavior

We tested the hypothesis that metabolism of thyroid hormones by the brain o f Atlantic salmon (Salmo salar) changes when rheotactic behavior reverses d uring parr-smolt transformation (PST). We measured brain and liver thyroxin e (T-4) and 3,5,3'-triiodo-L-thyronine (T-3) outer-ring deiodination (ORD) and inner-ring deiodination (IRD) activities and plasma T-4 and T-3 levels in Atlantic salmon held under natural photoperiod infresh water at 10degree sC in the spring of 1993 and 1994. We also measured changes in T-4, T-3, an d cortisol levels during the change in rheotactic behavior. Condition facto r decreased while salinity tolerance improved from mid-March to late April. The turbidity-induced transition from upstream to downstream swimming occu rred in mid to late April. The main changes inbrain deiodination were reduc ed T3IRD (1993 study) and elevated T4ORD (1994 study). In both years, a hig h ratio of T4ORD/T3IRD activities in the brain indicated an increased poten tial for T-3 production in the brain during advanced PST. Liver deiodinatio n profiles differed between years, but during advanced PST the low T4ORD ac tivity and low T4ORD/T3IRD activity ratio suggesteda low potential for hepa tic, and hence systemic, T-3 production. However, plasma T-4 was increased in downstream swimmers at 1d (1993) and 4h (1994) after the turbidity incre ase. Since at this time brain deiodination pathways were poised towards T-3 production, the surge in plasmaT(4) would likely increase local T-3 format ion in brain. We conclude that during PST there is no major change in hepat ic deiodination and hence probably no major change in systemic T-3 availabi lity. But deiodination properties in brain during late PST indicate the pot ential for local T-3 formation. This may be significant when plasma T-4 inc reases at the time of downstream migration.