Ontogenesis of 3,5,3′-Triiodothyronine Receptors in Neonatal Rat Brain: Dissociation between Receptor Concentration and Stimulation of Oxygen Consumption by 3,5,3′-Triiodothyronine*

Abstract

Studies were undertaken to define the concentration of T3 nuclear receptors in the brain of the developing rat. Efforts were made to correlate such concentrations to the level of circulating T3 and to the period during which T3 exerts its maximal effects on maturation of the central nervous system (CNS). Moreover, the effect of T3 on O2 consumption by slices of neonatal and adult brain was tested in order to determine whether or not neonatal brain, unlike adult brain, is responsive to T3 by this criterion. Receptor sites were demonstrated in fetal whole brain with a binding capacity of 0.17 ± 0.02 ng T3/mg DNA, a value only slightly lower than in adult brain (0.22 ± 0.03 ng T3/mg DNA). Two days after birth, the binding capacity had risen sharply to 0.38 ± 6.04 ng T3/mg DNA. During the subsequent 4 weeks, the binding capacity fell to adult levels. Plasma T3 concentrations were exceedingly low in the fetal serum (0.08 ± 0.01 ng/ml) despite the near normal nuclear binding capacity of brain. A rapid increase ...