Cortisol receptors and inducibility of glutamine synthetase in embryonic retina.

Glutamine synthetase (GS) is a marker enzyme for Müller glia cells in neural retina. In chick embryo retina GS begins to increase sharply on the 16th day of development, but can be precociously induced by premature supply of the inducer, cortisol, already on the 8th day. At this stage GS inducibility is low, but it increases progressively with embryonic age. We investigated whether there was a corresponding age-dependent increase of cortisol-binding molecules (cortisol receptors) and found that their level is highest in the early retina and decreases with development. In light of this inverse relationship, we examined whether functional characteristics of these receptors change with age, but detected no differences. In in vitro tests, receptors from older retina translocated cortisol into nuclei from young retina, and vice versa, with similar effectiveness. Also, cortisol receptors from liver cells (which differ from retina receptors) can translocate the hormone into retina nuclei, and vice versa. These findings indicate that translocation of cortisol receptors is neither tissue-specific or age-dependent, nor is it conditional on the total amount of receptors normally present in cells. Therefore, the age-dependent increase of GS inducibility in embryonic retina cannot be directly related to quantitative or functional differences of cortisol receptors and is evidently controlled primarily at the gene level. The very large amount of cortisol-binding molecules in early embryonic retina raises the possibility that they play some role in early differentiation of retina cells unrelated to hormone binding.