Nuclear T3 receptor (NTR) have been characterized in separated cultures of 
neurons and astrocytes. Scatchard analysis indicated the presence of a single 
class of high-affinity sites in both cell lines. The apparent equilibrium 
association constant ranged from 1.80 +/- 0.41 X 10(10) M-1 to 3.27 +/- 0.74 +/- 
10(10) M-1 in neurons and from 1.01 +/- 0.09 to 1.80 +/- 0.73 X 10(10) M-1 in 
astrocytes depending on the time in culture. In neurons, the maximal binding 
capacity (MBC) increased from 0.049 +/- 0.008 ng T3/mg DNA to 0.328 +/- 0.052 ng 
T3/mg DNA between 3 and 12 days of culture. In astrocytes, the changes in MBC 
were less pronounced ranging from a minimum of 0.095 +/- 0.024 ng T3/mg DNA at 
the 7th day of culture to a maximum of 0.198 +/- 0.048 ng T3/mg DNA at the 21st 
day. The relative binding affinity of the receptor for thyroid hormone analogs 
was in the order TRIAC greater than L-T3 greater than D-T3 greater than L-T4 in 
both cell lines. These results show that nuclear T3 receptors similar to those 
found in vivo are present in primary cultures of both astrocytes and neurons.