Effects of glutamate and its analogs on intracellular calcium levels in the developing retina

Stimulation of neuronal cells by the excitatory amino acid, glutamate, often leads to a rise in cytosolic free calcium concentration ([Ca2+]i), which can affect cell survival and differentiation. The early appearance of endogenous glutamate in the embryonic rabbit retina suggests that it may be involved in intercellular signalling during development. Thus, the effect of glutamate on the [Ca2+]i of cells in the fetal and neonatal rabbit retina was examined using Ca2+ imaging techniques, which enabled the responses of large numbers of morphologically identified classes of cell to be compared directly. Ganglion cells and amacrine cells, the first retinal neurons to differentiate, showed a rise in [Ca2+]i in the presence of glutamate from the earliest age studied (embryonic day 20; E20). These responses were mediated by non-NMDA (non-N-methyl-D-aspartate) receptors. NMDA stimulated ganglion cells and amacrine cells only several days later, at about E24. Moreover, whilst most, if not all, putative ganglion cells responded to NMDA, only a subset of putative amacrine cells were sensitive to NMDA throughout development. Photoreceptors, bipolar cells, horizontal cells, and Müller cells differentiate later than the ganglion cells and amacrine cells. Between E20 and birth, cells in the ventricular zone are largely the precursors of these cell types. During this period, 50-60% of ventricular cells responded to glutamate with an increase in [Ca2+]i, upon activation of ionotropic non-NMDA receptors. At no age studied were these ventricular cells, or their differentiated counterparts, stimulated by NMDA. After birth, most cells in the inner nuclear layer were sensitive to non-NMDA receptor agonists, but photoreceptors showed no response. Taken together, the results suggests that NMDA and non-NMDA receptors may adopt separate roles during retinal development, and that non-NMDA receptors, rather than NMDA receptors, may be involved in developmental processes in the ventricular zone.