Oral supplementation with melatonin reduces oxidative damage and concentrations of inducible nitric oxide synthase, VEGF and matrix metalloproteinase 9 in the retina of rats with streptozotocin/nicotinamide induced pre-diabetes

Hyperglycemia mediated oxidative stress and pro-angiogenic molecules such as vascular endothelial growth factor (VEGF) and matrix metalloproteinase 9 (MMP9) are considered important for diabetic retinopathy onset and progression. Melatonin is a pineal hormone that regulates circadian and seasonal rhythms and most likely is involved in regulating glucose metabolism. We aimed to evaluate the potential benefit of melatonin supplementation to the pre-diabetic retina by assessing melatonin effects on lipid peroxidation (thiobarbituric acid reactive substances, TBARS), protein oxidation (advanced oxidation protein products, AOPP) and concentrations of inducible nitric oxide synthase (iNOS), VEGF and MMP9 in the retina of rats with pre-diabetes. Pre-diabetes was induced by streptozotocin (45 mg/kg, i.p.) following nicotinamide injection (110 mg/kg, i.p.). Beside mild hyperglycemia, lower serum insulin, increased fructosamine and lower HDL cholesterol, the present study demonstrated decreased serum melatonin in pre-diabetic rats, as well as, increased concentration of retinal TBARS, AOPP, iNOS, VEGF, and MMP9. Oral supplementation with melatonin (85 μg/animal/day) caused melatonin and HDL cholesterol levels to rise in treated rats and reduced levels of fasting serum glucose and fructosamine. It also affected serum insulin and quantitative insulin sensitivity check index (QUICKI) in treated groups but had no significant effect on non-fasting glucose. Finally, supplementation with melatonin reduced concentrations of TBARS, AOPP, iNOS, VEGF, and MMP9 in significant level, thereby exerting an overall positive effect on oxidative stress and pro-angiogenic signaling in the pre-diabetic retina. Thus, oral melatonin might be considered in an early treatment or in the prevention of retinal changes associated with pre-diabetes.