Effects of feeding and photocycle on diel changes in plasma thyroid hormone levels in rainbow trout, Salmo gairdneri

In rainbow trout held in the laboratory within a narrow temperature range on a 12L:12D photocycle and fed once during the photophase there was a marked diel change in plasma L-thyroxine (pT4) and a less pronounced change in plasma 3,5,3′-triiodo-L-thyronine (pT3). Both photocycle and feeding regimes were experimentally altered to determine their influence. During starvation there was no significant diel change in plasma T4 in fish held in continuous darkness or under a phase-shifted (12D:12L) photocycle and minor fluctuations under a normal (12L:12D) photocycle, although phase-shifting did generally depress pT4. Feeding caused a tonic elevation in pT4, a marked postprandial surge in pT4 and facilitated a T4 surge due to light in the early photophase. Under most of these conditions changes in pT3 were less pronounced and peak pT3 levels usually followed the pT4 peaks, suggesting peripheral T3 derivation from T4. In conclusion, trout held at constant temperature show a diel profile in pT4 determined by an interaction between the feeding and photocycle regimes and which in turn probably influences delayed and more modest diel changes in pT3.