GLUTAMINE-SYNTHETASE (GS) ACTIVITY AND SPATIAL AND TEMPORAL PATTERNS OF GS EXPRESSION IN THE DEVELOPING CHICK RETINA - RELATIONSHIP WITH SYNAPTOGENESIS IN THE OUTER PLEXIFORM LAYER

The profile of glutamine synthetase (GS) activity in the neural retina of chicken embryos and adults was studied alongside the in vivo spati o-temporal patterns of generation and morphogenesis of Muller cell and of retinal synaptogenesis. The rise of GS activity during development is not related to Muller cell differentiation but to synaptogenesis i n the outer plexiform layer (opl). GS expression was investigated by i mmunoreaction with GS-specific antiserum. Three spatial gradients of d ecreasing labeling were observed between embryonic (E)15 and E18, from central to peripheral retina, dorsal to ventral, and temporal to nasa l, which are in spatio-temporal relationship with synaptogenesis in th e opl. GS is localized in Muller cells and apparently also in a popula tion of astrocyte-like glial cells, located in the ganglion cell layer throughout the retina. Precocious induction by hydrocortisone, in ovo , at E10, does not show the spatial pattern of GS immunoreactivity obs erved in control retinas at the time of natural induction (E15). We al so show that dissociated (noncultured) Muller cells of E18-20 retinas, to which only photoreceptors or photoreceptors and neurons remain joi ned, maintain an immunodetectable level of GS, while those in isolated state lose GS immunoreactivity rapidly. Our results suggest that the induction of GS expression might be mediated by Muller cell-neuron int eractions at the opl and also perhaps at the outer nuclear layer (onl) . An analysis of our results and those of previous authors suggests th at the level of GS in differentiated Muller cells could be determined by conjoint cell interactions at the onl, opl, and inner plexiform lay er. (C) 1998 Wiley-Liss, Inc.