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

• Published in: Canadian Journal of Zoology
• Main Authors: Specker, Jennifer L, Eales, J Geoffrey, Tagawa, Masatomo, Tyler, III, William A
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2000
• Subjects: Atlantic salmon; Salmo salar
• Online Access: http://dx.doi.org/10.1139/z99-258; http://www.nrcresearchpress.com/doi/pdf/10.1139/z99-258

We tested the hypothesis that metabolism of thyroid hormones by the brain of Atlantic salmon (Salmo salar) changes when rheotactic behavior reverses during parr-smolt transformation (PST). We measured brain and liver thyroxine (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 in fresh water at 10°C in the spring of 1993 and 1994. We also measured changes in T 4 , T 3 , and cortisol levels during the change in rheotactic behavior. Condition factor decreased while salinity tolerance improved from mid-March to late April. The turbidity-induced transition from upstream to downstream swimming occurred in mid to late April. The main changes in brain deiodination were reduced T 3 IRD (1993 study) and elevated T 4 ORD (1994 study). In both years, a high ratio of T 4 ORD/T 3 IRD activities in the brain indicated an increased potential for T 3 production in the brain during advanced PST. Liver deiodination profiles differed between years, but during advanced PST the low T 4 ORD activity and low T 4 ORD/T 3 IRD activity ratio suggested a low potential for hepatic, and hence systemic, T 3 production. However, plasma T 4 was increased in downstream swimmers at 1 d (1993) and 4 h (1994) after the turbidity increase. Since at this time brain deiodination pathways were poised towards T 3 production, the surge in plasma T 4 would likely increase local T 3 formation in brain. We conclude that during PST there is no major change in hepatic deiodination and hence probably no major change in systemic T 3 availability. But deiodination properties in brain during late PST indicate the potential for local T 3 formation. This may be significant when plasma T 4 increases at the time of downstream migration.