Nuclear triiodothyronine receptors in the developing rat brain.

This study examines whether the high sensitivity of the developing brain to thyroid hormones and the purported decline in sensitivity in adulthood, are correlated with changes in the density and affinity characteristics of specific nuclear T3 receptors. The authors have found that the nuclei of cerebral hemispheres have a high density of T3 receptors at birth (212 +/-28 X 10(-17) mol/microgram DNA) which declines to adult levels by the end of the second postnatal week (115 +/- 7 X 10(-17) mol/microgram DNA), remaining at this level until 6 months of age. Even though no significant changes were detected in the equilibrium dissociation constant (Kd) during the early period of development, comparison neonatal with the adult brain reveals a decrease in Kd (neonatal, 3.9 X 10(-10) M; adult, 2.3 X 10(-10) M). In the developing animal, neonatal thyroidectomy increased the number of binding sites in the nucleus by 36--44%. It is concluded that the high number of nuclear T3 receptors in the first week of postnatal life is correlated with the high dependence of brain tissue on thyroid hormones and that the decline in brain sensitivity may be associated with the decline in nuclear T3 receptors. The high affinity and density of nuclear receptors in adult brain tissue relative to the developing brain and liver, respectively, point to a continued regulatory role of thyroid hormones in brain.