RETINAL GANGLION CELLS ARE RESISTANT TO AMPA RECEPTOR MEDIATED EXCITOTOXICITY

Glaucoma is an age-related disease that affects nearly 70 million people worldwide. It is characterized by damage to the cells in the back of the eye which eventually die and cause gradual vision loss. The mechanism to how glaucoma occurs is yet unknown but there are many speculations. A protein molecule called the AMPA receptor is speculated to play a role in glaucoma by causing the death of these cells in the back of the eye. In our study, we are isolating the cells from the back of the eye of rats to study the role of the AMPA receptor and how it truly functions. Understanding basic functions of this protein molecule can one day help us develop drugs targeting AMPA receptors and therefore possibly protecting the dying cells in glaucoma. Purpose (a): The ionotropic glutamate receptors (iGluR) have been hypothesized to play a role in glaucoma pathogenesis by mediating excitotoxic death of retinal ganglion cells (RGC). Previous studies on iGluR in the retina have been focused on two broad classes of receptors: NMDA and non-NMDA receptors including the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) and Kainate receptor. In this study, we examined the specific excitotoxic effects of activation of the AMPAR in RGCs in-vitro. Methods (b): Purified rat RGCs were isolated from P3-P5 Sprague-Dawley rats by a double immunopanning technique using an antibody to Thy 1.1. RGCs were cultured for 7 days before s-AMPA (100μM) treatments. s-AMPA excitotoxicity was determined by Caspase3/7 luciferase activity assay, immunoblot analysis for α-fodrin and Live (calcein AM)/Dead (ethidium homodimer-1) assay. Gap-43 expression was assessed by immunocytochemistry. Results (c): Treatment of cultured RGCs with s-AMPA (100μM) for 24, 48 and 72h, both in the presence and absence of trophic factors (BDNF and CNTF), did not alter caspase 3/7 activity and cleavage of α-fodrin (neuronal apoptosis marker), compared to untreated controls. A significantly higher (p<0.05) cell survival of RGCs (85.3±1.5% alive cells) was observed after a 72h treatment with 100μM s-AMPA compared to control untreated RGCs (74.8±3.1% alive cells). Quantification of s-AMPA (100μM) – mediated excitotoxicity in purified RGCs incubated for 24h in an oxygen/glucose deprived (0.5% oxygen) medium demonstrated no statistically significant differences in cell survival compared to control RGCs maintained under either normoxia or hypoxia. Additionally, immunocytochemical analysis showed increased GAP-43 staining in RGCs after 24h of treatment with s-AMPA (100μM). Conclusions (d): These results indicate that purified RGCs in-vitro are not susceptible to AMPA excitotoxicity as previously hypothesized. Activation of AMPAR increased GAP-43 expression, suggesting AMPAR could possibly increase neurite outgrowth. The ability of AMPA receptors to promote neuroprotection of RGCs remains to be confirmed.