Regulation of melatonin in the retinae of guinea pigs: Effect of environmental lighting

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Abstract
We studied the effect of environmental lighting on the level of melatonin in the retinae of guinea pigs. Male pigmented guinea pigs were adapted to 12-hour light: 12-hour dark (with lights on at 0600 hours) for 2 weeks. In experiment 1, two groups of animals were decapitated at 1600 and 0400 hours. Melatonin was extracted from retinae and quantified by radioimmunoassay. A diurnal rhythm of retinal melatonin level per pair of retinae was demonstrated. In experiment 2, one group was subjected to an extended dark period (lights remained off after 0600 hours) while the control group remained on the normal photoperiod on the day of sacrifice. Both groups were killed at 1200 hours. The retinal melatonin level in the group under extended dark period was significantly higher (P < 0.05) than that of the control. In experiment 3, one group was exposed to an extended light period (lights remained on after 1800 hours) and the control group remained on the normal lighting schedule. Both groups were killed at 2400 hours. The retinal melatonin level in the group exposed to an extended light period was significantly lower (P < 0.05) than that of the control. Thus, darkness induces an increase while light causes a decrease in retinal melatonin level.