Uptake of radiolabelled triiodothyronine in the brain of steelhead trout (Salmo gairdneri) during parr-smolt transformation: Implications for the mechanism of thyroid activation of olfactory imprinting

Abstract Two groups of 13-month-old trout — treated with either TSH (induced smolts) or saline (parr) — were examined for in vivo uptake of radiolabelled triiodothyronine ( 125 I-T 3 ) into brain nuclei. This was accomplished by injecting a tracer dose of 125 I-T 3 either alone or with a 1000-fold excess of nonradioactive T 3 and measuring the amount of radioactivity that accumulated in isolated brain nuclei in each case. Uptake of 125 I-T 3 was reduced by 80–90% for both TSH and saline groups receiving nonradioactive T 3 when compared with their counterparts receiving radiolabelled T 3 only. This result is consistent with the suggestion that trout brain contains saturable nuclear receptors for T 3 . Similar experiments indicate gill, integument and olfactory epithelium may also be target cells that contain T 3 receptors. In gut and muscle no difference in uptake of 125 I-T 3 was seen between fish with or without nonradioactive T 3 , suggesting that these tissues do not contain T 3 receptors. All of the putative target tissues contained consistently more radioactivity and all nontarget tissues consistently less radioactivity in the TSH induced smolts than in the saline presmolts. This result is consistent with the conjecture that an ontogenetic change occurs in receptor binding kinetics during smolt transformation; i.e., T 3 receptor concentration and/or binding affinity may have increased in the target tissues of the TSH group so that less T 3 would be available for uptake by nontarget tissues.