ACh receptors link two signaling pathways to neuroprotection against glutamate-induced excitotoxicity in isolated RGCs

Previous studies have reported that activation of nicotinic acetylcholine (ACh) receptors (nAChRs) on cultured pig retinal ganglion cells (RGCs) has a neuroprotective effect against glutamate-induced excitotoxicity. However, the mechanism linking nAChRs to neuroprotection is unknown. Here, we tested the hypothesis that signaling cascades involving p38 mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) -> Akt are involved in linking activation of nAChRs to neuroprotection in isolated pig RGCs. In ELISA studies, regulation of phosphorylated p38 MAPK and Akt were analyzed after inducing excitotoxicity or neuroprotection in the presence and absence of specific inhibitors for p38 MAPK and PI3K. ELISA results demonstrated that ACh significantly increased phosphorylated Akt and decreased p38 MAPK. Glutamate increased phosphorylated p38 MAPK but had no significant effect on phosphorylated Akt. Other ELISA studies using p38 MAPK and PI3K inhibitors also supported the hypothesis that ACh up-regulated Bcl-2 levels down-stream from PI3K and Akt, whereas glutamate down-regulated Bcl-2 levels downstream from p38 MAPK. RGC survival was subsequently assessed by culturing RGCs in conditions to induce excitotoxicity or neuroprotection in the presence or absence of specific inhibitors of p38 MAPK or PI3K. The p38 MAPK inhibitor significantly decreased the number of RGCs that died by glutamate-induced excitotoxicity but had no effect on the number of cells that survived because of ACh-induced neuroprotection. PI3K inhibitors significantly decreased cell survival caused by ACh-induced neuroprotection but had no effect on cell death caused by glutamate-induced excitotoxicity. These results demonstrate that glutamate mediates excitotoxicity through the p38 MAPK signaling pathway and that ACh provides neuroprotection by stimulating the PI3-K -> Akt -> Bcl-2 signaling pathway and inhibiting the p38 MAPK -> Bcl-2 pathway.