d-Serine enhancement of NMDA receptor-mediated calcium increases in rat retinal ganglion cells.

NMDA receptor (NMDAR) activation is enhanced by d-serine or glycine acting at a specific binding site. Previous work has shown d-serine enhancement of NMDAR currents in retinal ganglion cells. One of the major functions of most NMDA channels is to permit calcium influx into cells. We show that d-serine enhances glutamate-induced calcium responses in immunopanned retinal ganglion cells. This effect was specific to NMDA receptors as similar results were found with NMDA, but not kainate, and was reduced or blocked by modulators of the NMDAR coagonist binding site. d-Serine and glycine enhanced glutamate-induced calcium responses in a dose-dependent manner and at equimolar concentrations there was no difference in the efficacy of the coagonists. In isolated retinas NMDA-induced calcium responses were enhanced by d-serine coapplication in 46% of ganglion cells. Endogenous d-serine degradation by treatment with d-amino acid oxidase caused a approximately 45% decrease in the NMDA-induced response that could be reversed by coapplication with d-serine. d-Serine and glycine were equally effective in enhancing glutamatergic calcium responses. Endogenous d-serine contributes to NMDAR activation in retinal wholemounts and some but not all retinal ganglion cells may experience saturating levels of d-serine or glycine.