EARLY DEVELOPMENT OF GABA-LIKE IMMUNOREACTIVE CELLS IN THE RETINA OF TURTLE EMBRYOS

Gamma aminobutyric acid (GABA) is one of the earliest neuroactive subs tances appearing in the developing central nervous system. The distrib ution and the time course of the appearance of GABA-like immunoreactiv ity in the retina of the turtle Emys orbicularis were investigated fro m embryonic stage 13 to hatching. The first GABA-like immunoreactive c ells were observed at stage 14. These cells were located in both the s cleral third of the neuroblastic layer and the inner layers of the ret ina. They were identified as presumptive immature horizontal cells and amacrine cells, respectively. The observation of numerous labelled fi bers in the nerve fiber layer suggests that some of the GABA-like immu noreactive cells in the inner layers were ganglion cells. The developm ent of GABA-like immunoreactive cells followed a gradient of maturatio n from central to peripheral retina. At hatching, the central retina a ppeared nearly morphologically mature. In conclusion, GABA is present before the morphofunctional maturation of the retina and this precocio us existence supports the idea of its involvement in a neurotrophic ro le preceding the establishment of synaptic connections and neurotransm itter function.