melatonin reduces cataract formation and aldose reductase activity in lenses of streptozotocin-induced diabetic rat

background: the relationship between the high activity of aldose reductase (ar) and diabetic cataract formation has been previously investigated. the purpose of the present study was to determine the preventing effect of melatonin on streptozotocin (stz)-induced diabetic cataract in rats. methods: 34 adult healthy male sprague-dawely rats were divided into four groups. diabetic control and diabetic+melatonin received a single dose of stz (50 mg/kg, intraperitoneally), whereas the normal control and normal+melatonin received vehicle. the melatonin groups were gavaged with melatonin (5 mg/kg) daily for a period of 8 weeks, whereas the rats in the normal control and diabetic control groups received only the vehicle. the rats’ eyes were examined every week and cataract formation scores (0-4) were determined by slit-lamp microscope. at the end of the eighth week, the rats were sacrificed and markers of the polyol pathway and antioxidative (glutathione, gsh) in their lens were determined. the levels of blood glucose, hba1c and plasma malondialdhyde (mda), as a marker of lipid peroxidation, were also measured. results: melatonin prevented stz-induced hyperglycemia by decreased blood glucose and hba1c levels. slit lamp examination indicated that melatonin delayed cataract progression in diabetic rats. the results revealed that melatonin feeding increased the gsh levels, decreased the activities of ar and sorbitol dehydrogenase (sdh) and sorbitol formation in catractous lenses as well as plasma mda content. conclusion: in summary, for the first time we demonstrated that melatonin delayed the formation and progression of cataract in diabetic rat lenses.