Effect of cold exposure on thyroid hormone metabolism and nuclear binding in rat brain

In this study we examined whether adult rat brain tissue (cerebral hemispheres) would under cold exposure respond with changes in the local metabolism and nuclear binding of thyroid hormones (T3, T4). Adult, control rats kept at 22 degrees C and cold exposed (4 degrees C, 20 h) rats were injected with trace of 125I-T4 or 125I-T3 returned to their respective environment and sacrificed four hours later. The radioactive hormonal forms were identified and quantified in the cytoplasmic and nuclear fractions. It was found that in cold exposed rats injected with 125I-T4, the total cytoplasmic radioactivity was higher than that of controls. This increase was not associated with 125I-T4 but it reflected an increase (88 %) in its deiodination product 125I-T3 (125I-T3 (T4)). Although total cytoplasmic 125I-T4 did not change, there was a decrease (28%) in its protein free cytoplasmic fraction. 125I-T3 (T4) and 125I-T4 bound to the nuclear fraction were found to decrease by 58 and 46% respectively. Cold exposed animals injected with 125I-T3 also showed an increase in cytoplasmic 125I-T3 (81%) and a decrease in 125I- (40%) whereas 125I-T3 bound to the nuclear fraction decreased by 64%. These results indicate that cold exposure of rats decreases brain local T3 metabolism and nuclear binding while it does not effect local T4 metabolism.