3.3. GLP-1 Eyedrops Protect from Oxidative Stress by Increasing the Protein Levels of Glutathione Reductase, Glutathione Peroxidase and Copper–Zinc and Manganese Superoxide Dismutases (CuZnSOD and MnSOD) in Diabetic Retinas. This evidence concerns the gene GSR and diabetes mellitus.