Changing patterns of spatial buffering of glutamate in developing rat retinae are mediated by the Muller cell glutamate transporter GLAST

The patterns of expression of the glutamate transporter GLAST were compared with the patterns of uptake of exogenous D-aspartate, which is a substrate for all glutamate transporters. At postnatal day 0, fine radial processes and end feet of presumptive Muller cells were weakly immunoreactive for GLA ST. At postnatal day 3, intense labelling was associated with astrocytes en veloping newly formed blood vessels on the vitread surface of the retina. B etween postnatal days 7 and 10, there was a rapid increase in the intensity of labelling in the Muller cells but clear stratification of GLAST-immunor eactive processes in the inner plexiform layer was not observed until postn atal day 14. By comparison, D-aspartate uptake was initially associated wit h a wide variety of cellular elements including most neuroblasts, presumpti ve Muller cells, and astrocytes associated with blood vessels but was absen t from the somata of many neurons in the ganglion cell layer and amacrine c ell layer. There was a gradual contraction in the numbers of cells that wer e able to take up D-aspartate, such that, by adulthood, uptake was restrict ed mainly to Muller cells and astrocytes. We conclude that, during early re tinal development, the low levels of GLAST expression by Muller cells permi t D-aspartate, and by inference, glutamate, to permeate the retina freely, thus allowing uptake by other glutamate transporters on other cell types. A s the retina matures, increased expression of GLAST by Muller cells restric ts the access of D-aspartate to other cellular compartments in the retina. This changing pattern of spatial buffering of glutamate by GLAST probably h as significant implications regarding our understanding of the role of glut amate during processes such as retinal synaptogenesis.