INHIBITORY EFFECT OF MELATONIN ON CATARACT FORMATION IN NEWBORN RATS - EVIDENCE FOR AN ANTIOXIDATIVE ROLE FOR MELATONIN

We evaluated the inhibitory effect of melatonin, a recently discovered scavenger of free radicals, on cataract formation in the newborn rat. The glutathione synthesis inhibitor, buthionine sulfoximine (BSO) (3 mmol/kg), was intraperitoneally injected into newborn rats for 3 conse cutive days starting on day 2 after birth. These glutathione depleted rats develop cataracts. Melatonin (4 mg/kg) was injected intraperitone ally into half of the rats once a day beginning at day 2 after birth; the other half of the animals received solvent daily. The incidence of cataract was observed on day 16, after the eyes of the newborn animal s had opened. Both reduced glutathione (GSH) and oxidized glutathione (GSSG) levels were measured. Cataracts were observed in all animals (1 8/18) treated with BSO plus solvent. The incidence of the cataract in the animals cotreated with melatonin was only 6.2% (1/15). Total lenti cular glutathione (GSH + GSSG) levels in BSO only treated rats were re duced by 97%. The total glutathione in the lens of the BSO plus melato nin group was significantly higher (by 3%) than that of the BSO only g roup. The percentage of the total glutathione as GSSG for the BSO plus solvent group was higher than the control value. Cotreatment of BSO i njected rats with melatonin (4 mg/kg/day) clearly reduced cataract for mation proving that it is directly or indirectly protective against ox idative stress which accompanies glutathione deficiency. The inhibitor y effects of melatonin on cataract formation in this study could be du e to melatonin's free radical scavenging activity or due to its stimul atory effect on glutathione production.