Development of the glutamate system in rabbit retina.

We have investigated two characteristics of the glutamate system in the developing rabbit retina. 1) Glutamate immunoreactivity was observed at birth within developing processes of four cell types; two of which, photoreceptors and ganglion cells, are known to be glutamatergic in the adult. Two other cell types, type A horizontal cells and amacrine cells, are immunoreactive to both glutamate and GABA at birth, suggesting that endogenous pools of glutamate in GABAergic neurons serve as precursor for GABA synthesis. Thus it appears that endogenous glutamate pools are present within neurons prior to synaptogenesis as part of the early expression of either the glutamate or GABA transmitter phenotype. 2) Analysis of 3H-glutamate metabolism during retinal development showed that rapid conversion of glutamate to glutamine does not occur until the second postnatal week, coincident with the expression of Muller (glial) cell activity. In the absence of glial metabolism in the neonate, extracellular concentrations of glutamate remain relatively high and are likely to have major effects on neuronal maturation.