Serum affects the characteristics of excitatory amino acid-binding sites on Müller cells

The binding of [3H]L-aspartate to membranes obtained from primary cultures of chick retinal Müller cells (glia) was studied Cells seeded in low-serum-containing medium (1%) and maintained in this condition showed an increased number of binding site from 1 to 5 days in vitro (DIV), when compared with controls cultured in medium containing 10% serum; these changes were not reversed by the addition of 10% serum after 48 h in vitro. Increased binding at this age was due to the expression of a low affinity binding system, competitively inhibited by the glutamate uptake blocker L-aspartate-beta-hydroxamate, suggesting that high serum might inhibit the expression of uptake sites at precise maturation stages. Experiments showed the effect was due to a thermolabile serum component. The increase in binding sites is parallel in time to both an increase in aspartate uptake and the initiation of synaptogenesis in the whole retina. Our results suggest that the presence of serum at defined stages in retinal development, could result in the elevation of extracellular glutamate and the concomitant excitotoxic death of neuronal cells, due to a decreased glutamate uptake by glial cells.