Direct Action of Thyroxine on the Trout Pituitary in vitro

ALTHOUGH it is established that a negative feed-back system operates between the thyroid and pituitary in teleosts 1 , the mechanism controlling the activity of the pituitary thyrotrophs has scarcely been examined. In mammals where the problem has been extensively investigated, a normal or high level of thyrotrophic activity seems to depend on stimulation from the hypothalamus and requires an intact hypothalamo-pituitary link 2 ; a reduction of thyrotrophic activity can largely be attributed to the inhibitory action of thyroxine acting both on the hypothalamus and also directly at the pituitary level 3,4 In the teleost Poecilia (Mollienesia) formosa , experiments, suggest, on the contrary, that the thyrotrophs possess an intense autonomous activity which is normally suppressed by the hypothalamus 5 . Whether or not thyroxine also has a direct inhibitory effect on their activity is not known. The present work investigates the nature of the hypothalamic influence and the effect of thyroxine on the trout pituitary, using an organ-culture technique.