EARLY APPEARANCE AND TRANSIENT EXPRESSION OF PUTATIVE AMINO-ACID NEUROTRANSMITTERS AND RELATED MOLECULES IN THE DEVELOPING RABBIT RETINA - AN IMMUNOCYTOCHEMICAL STUDY

We have studied, by immunocytochemistry, the ontogeny of GABA, glycine , glutamate, glutamine, and taurine-containing cells in the rabbit ret ina. Amacrine cells show GABA immunoreactivity by embryonic day 25 (E2 5) and throughout postnatal life. By contrast, ganglion cells and hori zontal cells are only transiently GABA-immunoreactive (-IR); few appea r GABA-IR by the third postnatal week. At maturity, glycine is present in amacrine cells and in some bipolar cells. During development, puta tive ganglion cells transiently contained glycine between E25 and post natal day 3 (P3), whereas immunolabelling in presumed amacrine cells a nd bipolar cells persists after birth. Ganglion cells, bipolar cells, photoreceptors, and some amacrine cells are glutamate-IR in the adult retina. Glutamate immunoreactivity first appears in the somata and pro cesses of cytoblastic cells by E20 and is prominent by E25. Surprising ly, ganglion cells are not strongly glutamate-IR until just before eye -opening, at postnatal day 10 (P10), coincident with the appearance of glutamine in their somata and in Muller glial cells. Bipolar cells ar e glutamate-IR before they or Muller cells contain high levels of glut amine (at P10). Glutamate immunoreactivity in photoreceptors is progre ssively restricted to the inner segments by eye-opening. At no stage a re presumed horizontal cells glutamate-IR or glutamine-IR, but some am acrine cells show glutamate- and glutamine-IR by P10. Taurine is local ized to photoreceptors and Muller glial in the adult retina. Some cyto blasts are taurine-IR at E20; with ensuing development, taurine labell ing becomes restricted primarily to Muller cells and photoreceptors; s ome putative bipolar cells may also be labelled. However, for a few da ys around birth, cells resembling horizontal cells, also show taurine immunoreactivity. The early appearance and often transient expression of these amino acids in retinal cells suggests that these neuroactive molecules may be involved in the structural and functional development of the retina.