Localization of glutamine synthase in the retina. Paraffin sections of mouse (A, B), rat (C, D, G-I), or human (E, F) retina fixed in 4% paraformaldehyde were reacted with anti-glutamine synthase (red fluorescence staining in B, D, G, I, and brown immuno-peroxidase reaction (using ABC kit and visualization with DAB product in F). Nuclei in some immunofluorescence experiments (A-D) were stained with DAPI (shown in cyan), and with nuclear fast red in E and F. On inspection at low magnification, anti- glutamine synthase reacted with a single population of cells extending from the ganglion cell layer (GCL) through the inner nuclear layer (INL). No signal was detected in controls either pre-incubated with 100 µg/mL of the immunizing peptide (A) or with pre-immune serum (C, E). The pattern of staining observed in all experiments is typicals. This finding was confirmed by co-localization (indicated by yellow in I) of glutamine synthase (red in G) with antoher marker of glutamine synthase (green in H).