Vitamin C (in the reduced form ascorbate or in the oxidized form dehydroascorbate) is implicated in signaling events throughout the central nervous system (CNS). In the retina, a high‐affinity transport system for ascorbate has been described and glutamatergic signaling has been reported to control ascorbate release. Here, we investigated the modulatory role played by vitamin C upon glutamate uptake and N‐methyl‐d‐aspartate (NMDA) receptor activation in cultured retinal cells or in intact retinal tissue using biochemical and imaging techniques. We show that both forms of vitamin C, ascorbate or dehydroascorbate, promote an accumulation of extracellular glutamate by a mechanism involving the inhibition of glutamate uptake. This inhibition correlates with the finding that ascorbate promotes a decrease in cell surface levels of the neuronal glutamate transporter excitatory amino acid transporter 3 in retinal neuronal cultures. Interestingly, vitamin C is prone to increase the activity of NMDA receptors but also promotes a decrease in glutamate‐stimulated [³H] MK801 binding and decreases cell membrane content of NMDA receptor glutamate ionotropic receptor subunit 1 (GluN1) subunits. Both compounds were also able to increase cAMP response element‐binding protein phosphorylation in neuronal nuclei in a glutamate receptor and calcium/calmodulin kinase‐dependent manner. Moreover, the effect of ascorbate is not blocked by sulfinpyrazone and then does not depend on its uptake by retinal cells. Overall, these data indicate a novel molecular and functional target for vitamin C impacting on glutamate signaling in retinal neurons. image

Dopamine and glutamate are critical neurotransmitters involved in light-induced synaptic activity in the retina. In brain neurons, dopamine D₁receptors (D₁Rs) and the cytosolic protein tyrosine kinase Src can, independently, modulate the behavior of NMDA-type glutamate receptors (NMDARs). Here we studied the interplay between D₁Rs, Src and NMDARs in retinal neurons. We reveal that dopamine-mediated D₁R stimulation provoked NMDAR hypofunction in retinal neurons by attenuating NMDA-gated currents, by preventing NMDA-elicited calcium mobilization and by decreasing the phosphorylation of NMDAR subunit GluN2B. This dopamine effect was dependent on upregulation of the canonical D₁R/adenylyl cyclase/cAMP/PKA pathway, of PKA-induced activation of C-terminal Src kinase (Csk) and of Src inhibition. Accordingly, knocking down Csk or overexpressing a Csk phosphoresistant Src mutant abrogated the dopamine-induced NMDAR hypofunction. Overall, the interplay between dopamine and NMDAR hypofunction, through the D₁R/Csk/Src/GluN2B pathway, might impact on light-regulated synaptic activity in retinal neurons.