Evolution of triiodothyronine nuclear binding sites in hypothalamic serum-free cultures: evidence for their presence in neurons and astrocytes.

[125I]Triiodothyronine (T3) nuclear binding was studied in hypothalamic cultures from fetal mouse grown in serum-free medium. In enriched neuronal cultures, the apparent dissociation constant of the binding does not change with time in vitro (7 X 10(-11) M), but the maximum binding capacity (MBC) doubles between day 7 and day 14 in vitro. We show here for the first time that homologous astrocyte cell cultures, devoid of neurons as checked by tetanus toxin binding, also display T3 nuclear binding, with the same affinity as neuronal cultures. However, their MBC is 3 times lower than that of neurons after a week in vitro, and increases more quickly thereafter.