TRIIODOTHYRONINE RECEPTOR COMPLEX IN DEVELOPING RAT-BRAIN AND PITUITARY

In vitro saturation analysis combined with nuclear 3,5,3'-triiodothyro nine (T3) quantification was used to examine the changes in T3 binding parameters in rat pituitary and cerebrocortical nuclei from fetal day 14 to postnatal day 20. T3 receptors were first detectable in neurona l, glial, and pituitary nuclei on fetal days 14, 17, and 18, respectiv ely. Thereafter T3 receptor concentrations in neuronal, glial, and pit uitary nuclei increased throughout the developmental period studied, r eaching maximal levels during neonatal life (1,129, 1,025, and 635 fmo l/mg DNA, respectively). T3 levels in pituitary, neuronal, and glial n uclei also increased during development there being a 35-, 34-, and 12 0-fold rise between fetal days 16-18 and the 20th postnatal day. Endog enous T3 receptor occupancy throughout the experimental period increas ed six- to ninefold in the three types of nuclei. The presence of T3 r eceptor complex in the pituitary and cerebrocortical nuclei during per inatal development lends further support to the hypothesis that T3 may be an important factor in determining the differentiation and develop ment of these cells.