Effect of triiodothyronine on glutamine synthetase in the developing rat retina

It has been previously demonstrated that triiodothyronine (T3) causes a precocious increase in glutamine syntehtase (GS) activity in the developing retina as measured by the glutamyltransferase (GT) reaction. To determine its distribution and the mechanism of its increased activity an immunohistochemical study was performed in retinas of 1 to 24-day-old rats given a subcutaneous injection of T3 on the day of birth. No difference was seen between T3-treated rats and controls on postnatal days 1 and 2. However, there was significantly less stain in T3-injected animals than in control animals especially in the pigment epithelium on day 5 and in Müller cells on days 8. By days 12 and 24 no difference was observed between experimental and control rats. We were thus unable to demonstrate increased synthesis of GS to correlated with its increased activity following T3 administration. On the contrary, we obtained evidence of decreased GS synthesis. It is suggested that T3 either causes both increased GT activity and decreased GS synthesis or that the T3-induced elevation of GS activity results in decreased synthesis of GS presumably through end-product inhibition.