3,5,3′-Triiodothyronine binding sites in synaptosomes from brain of chick embryo. Properties and ontogeny

In this study we have demonstrated the presence of specific 3,5,3′- l-triiodothyronine (T3) binding sites in the synaptosomes of chick embryo cerebral cortex and described their ontogeny. Scatchard analyses of binding data obtained with synaptosomal preparations from 17-day-old embryos revealed two T3 binding sites. The first site (N1) had a high affinity and low capacity since its dissociation constant ( K d) was 68 ± 1.3 nM T3 (mean ± S.D.; n = 3−5) and its maximal binding capacity ( B max) was 8.63 ± 1.59 ng T3/mg of protein, whereas the second site (N2) had a higher K d of 5.04 ± 0.5 μM T3 and a larger B max of 405 ± 49 ng T3/mg of protein. The relative affinity of the synaptosomal fraction for T3 and other analogs was the following: T3 > T4 (thyroxine) > d-T3 (3,5,3′- d-triiodothyronine) = TRIAC (triidothyroacetic acid) > rT3 (reverse T3) . Gel chromatography of the [ 125I]T3 labeled fraction revealed a partially saturable peak with an estimated MW of more than 100 kDa. The ontogenic pattern showed a progressive increase of K d and B max of N1, occuring mainly between the 12 and 19 days of incubation, and a marked fall, particularly of the B max, after hatching. The second site did not show any important variation during the embryogenesis. These data indicate the existence of specific T3 binding sites in synaptosomes from cerebral cortex of chick embryo, whose properties and ontogeny are completely different from those of the nuclear receptor. Furthermore these findings suggest that the synaptosomes could be another site of action of thyroid hormone where T3 could stimulate the development of specialized nervous structures or modulate the activity of various neurotransmitters during the development of chick embryo cerebral cortex.