Differentiation of ganglion cells and amacrine cells in the rat retina: correlation with expression of HuC/D and GAP-43 proteins.

In order to understand the development of retinal cells, we have studied the temporal expression of HuC/D protein in embryonic, postnatal and adult rat retina. During development and in the adult retina, practically all cell somata in the ganglion cell layer and the vast majority of conventional amacrine cells in the inner nuclear layer displayed HuC/D immunoreactivity. Most but not all ganglion cells expressed HuC/D at embryonic day 15, suggesting a delay between final mitosis and the initiation of HuC/D expression. Immunoreactivity for HuC/D was also evident in developing but not mature horizontal cells. Combined immunohistochemical visualization of HuC/D protein and the growth-associated protein (GAP-43) showed a distinct localization of GAP-43 in a specific compartment close to the somato-dendritic region of developing HuC/D-positive cell somata. The localization of GAP-43 immunoreactivity to a specific soma compartment became less evident during maturation. Immunoreactivity for HuC/D and GAP-43 was also discernible in horizontal cells at postnatal day 14. In the adult retina, most GAP-43 immunoreactivity was seen in the inner plexiform layer. Detailed analysis showed that HuC/D and GAP-43 expression is restricted to subsets of retinal neurons during development and in the mature retina. Thus, GAP-43 appears to be correlated with initial steps of differentiation and outgrowth of dendritic processes in HuC/D-positive ganglion and amacrine cells.