Thyroid hormone receptors of developing chick brain are predominantly in the neurons.

The relative concentration of the triiodothyronine (T3) receptors in the neuronal and glial nuclei of developing chick brain have been studied. Scatchard analysis indicate that the number of T3 binding sites in the neuronal nuclei increases from 400 to 1600 sites/nucleus between 7-11 day of embryonic development without any concomitant change in the level of glial nuclear receptors (130 - 200 sites/nucleus). Both sites are of high affinity (Ka = 1-3 x 10(9) M-1) at all ages examined. The abundance of the T3- receptors in the neuronal nuclei and the close coincidence of the period of rise in the level of these receptors in these nuclei (7-11 day) with that of maximal neuronal growth and synaptogenesis (7-13 day) suggest that the neurons are the primary site of action of T3 in the developing brain.