Postnatal developmental expression of glutamine and related amino acids in the rat retinas

Purpose. To evaluate postnatal developmental changes in the amounts of retinal glutamate, glutamine and GABA, and in the distribution of retinal glutamine in the rat. Methods. Free amino acids were extracted from rat retinas of different postnatal stages, and the concentrations of glutamate, glutamine and GABA were determined by HPLC. Also, anti-glutamine antibody was raised and an immunocytochemistry was performed with paraffin-embedded retinal sections in parallel with free amino acid analyses. Results. Glutamate occurred in high concentrations at the birth and showed a stable pool, while glutamine and GABA remained low until postnatal day 3 or 5, and gradually increased in the developing rat retinas. Glutamine immunolabeling was observed in the retinal pigment epithelium and in a subpopulation of presumed amacrine cells in the early postnatal days. It was also found in Muller cells and in some ganglion cells or displaced amacrine cells in the ganglion cells layer. Glutamine immunolabeling was transiently observed also in horizontal cells. Finally, the immunolabeling was dominant in the inner and outer plexiform layers in the adult retinas. Conclusions. Postnatal developmental increase in the levels of glutamine and GABA might be dependent on the maturation of neurons or glial cells that possess the activity of the key enzymes of each amino acid. It was suggested that an expression of glutamine immunolabeling can be a marker of neurons that utilize glutamine as a precursor for glutamate or GABA, and of Muller cell maturations in postnatal early stage of the retina, while it changes to demonstrate the locations of glutamine cycle in the retina with adult characteristics.