PROLONGED NMDA STIMULATION INDUCES NEUROPROTECTIVE PATHWAYS AND ENHANCES SURVIVABILITY OF PRIMARY RETINAL GANGLION CELLS

Purpose (a): Calcium influx through postsynaptic NMDA receptors has been shown to stimulate a number of key pro-survival genes; however, prolonged stimulation has been shown to have excitotoxic effects leading to apoptosis in neurons. Previous studies have shown a rapid dephosphorylation of CREB in primary hippocampal neurons treated for 1-2 h with100µM NMDA . It is hypothesized that the activation of CREB-specific phosphatases is one of the main pathways that cause apoptosis during NMDA excitotoxicity. The current study investigated the role of NMDA stimulation on the phosphorylation of CREB in primary RGCs, and assessed if NMDA overstimulation caused excitotoxic changes similar to those seen in primary hippocampal neurons. In addition, the occurrence of NMDA excitotoxicity in bipolar and photoreceptor cells was also investigated. Methods (b): Purification and culture of RGCs were performed by sequential immunopanning using Thy 1 antibody from P3-P7 Sprague-Dawley rats. Mixed retinal cultures that remained following isolation of RGCs from the retina were plated once the RGCs were separated and purified. Calcium imaging was used to measure the intracellular changes in calcium following treatment of cells with 100µM NMDA. Western blots were performed to determine signaling pathways linked to NMDA induced cell survival or excitotoxicity. Calcein AM and ethidium homodimer were used to quantify cell survival and cell death. Cells were also subjected to a trophic factor deprivation insult for 6 hours and 24 hours. Results (c): Treatment of primary RGCs with NMDA (100 µM) for 6h caused a greater than 2-3 fold induction of the transcription factor pCREB. MK801 (NMDA antagonist) completely abolished endogenous levels of pCREB and blocked NMDA induction of pCREB. NMDA (100 µM) treatment for 6 and 24 hrs under trophic factor deprivation, protected RGCs from trophic factor deprivation induced cellular death. The mixed retinal cultures (retinal cells without RGCs) had an opposite effect, where the levels of pCREB were diminished and the neurons died when treated with 100 µM of NMDA. Conclusions (d): The data suggests that NMDA signaling is essential for RGC survivability and blocking calcium ion influx through this receptor by the NMDA blocker, MK801 can be detrimental to RGC function and survival. These results also demonstrate that primary RGCs behave differently than other neurons in the retina, and are not susceptible to NMDA excitotoxicity.