Extracellular cyclic GMP and its derivatives GMP and guanosine protect from oxidative glutamate toxicity

► cGMP protected best from oxidative stress among 54 putative extracellular ligands. ► The degradation products GMP and guanosine are even more protective. ► Guanosine increases system xc− activity. ► Guanosine inhibits the toxic calcium influx during oxidative glutamate toxicity.