DEMONSTRATION OF PUTATIVE THYROID-HORMONE RECEPTOR IN THE BRAIN NUCLEI OF SINGI FISH, HETEROPNEUSTES-FOSSILIS (BLOCH)

A single injection of [125I]triiodothyronine (T3) with or without stable T3 in Singi fish, Heteropneustes fossilis (Bloch), showed that the fish brain has saturable binding sites and that the specific uptake is 60-70% higher than the nonspecific uptake. The distribution kinetics of [125I]T3 in the serum, whole brain, and brain nuclei after a single injection of the labeled hormone showed that the removal of [125I]T3 from the serum was very rapid with the t 1 2 of about 3.3 hr and the incorporation of the hormone into the brain and brain nuclei were very slow and achieve a maximal value after 4-6 hr of postinjection. The binding of [125I]T3 to the isolated brain nuclei of Singi fish was further studied in vitro. Binding was linearly increased with the increasing concentration of the DNA (nuclei). The binding achieved equilibrium between 15 and 20 min at 27° and was stable at least for 1 hr. The binding was reversible in the presence of excess unlabeled T3. Scatchard analysis showed only a single class of binding sites. The mean dissociation constant (Kd) is 2.15 ± 0.45 × 10-10 M and maximum binding capacity (MBC) is 0.044 ± 0.024 pmol/mg DNA. The relative binding affinities of thyroid hormone analogs for T3 sites were as follows: TRIAC > T3 > TETRAC > T4 > reverse T3 > T2. These findings were similar to those for other animals. Therefore, the nuclear binding sites in Singi fish brain, as demonstrated, may be regarded as thyroid hormone receptors. © 1991.