Melatonin as an antioxidant in retinal photoreceptors

Abstract

Dark-adapted, single photoreceptors isolated from the frog retina produce reactive oxygen species (ROS) after about 1 min of illumination with saturating light that we verified by their oxidation of preloaded dihydrorhodamine 123 (DHR) into the fluorescent rhodamine 123 (RHO). In this preparation we tested the antioxidant effects of vitamin E and of melatonin. Melatonin at picomolar and low nanomolar concentrations was determined to be 100 times more potent in inhibiting the light-induced oxidative processes than was vitamin E. On the contrary, both compounds exerted potent prooxidant effects at micromolar concentrations that is above the physiological levels of melatonin. This provides evidence that physiological concentrations of melatonin in a living cell may exert protective actions against a natural oxidant stimulus (light). This helps to define the functional role of endogenous melatonin in photoreceptors, which by their physiological characteristics, are among the marked producers of ROS in the organism.

Melatonin modulates visual function and cell viability in the mouse retina via the MT1 melatonin receptor

“…Indeed, previous studies have shown a possible role for retinal melatonin as a free radical scavenger within photoreceptors. In isolated photoreceptors from frog retina, melatonin was approximately100 times more potent than vitamin E in inhibiting light-induced oxidative processes (32) and melatonin can reduce the lipid peroxidation induced by nitric oxide in rat retinal homogenates (33). However, supraphysiological concentrations of melatonin may be needed for the antioxidant actions, and MT1 receptor activation appears to play a dominant role in maintaining cell viability.…”

Antioxidant protection in cultured corneal cells and whole corneas submitted to UV-B exposure

“…Melatonin is synthesized also in retinal photoreceptor cells and in the ciliary body of the eye and appears to exert protective actions against light-stimulated production of ROS [34][35][36]. In some tests, we compare the protective effect of pirenoxine and melatonin to that of a-tocopherol (physiological scavenger of hydroxyl radical) and thioctic acid (a scavenger molecule also able to neutralize superoxide anion) [37,38].…”

Retinal photoreceptors of Syrian hamsters undergo oxidative stress during streptozotocin-induced diabetes

“…Direct evidence of oxidative stress, however, has not yet been found in diabetic retinal photoreceptors, even though their high physiological production rate of reactive molecular species [3] and their abundant unsaturated fatty acids make these cells a principal substrate for lipid peroxidation.It has recently been reported that exposure of the outer segments of rod photoreceptor cells, isolated from the frog retina, to bright light of 485 nm rapidly induces oxidative reactions followed by irreversible cell damage [4]. The light-induced oxidative process depends on a transient state of activated rhodopsin which promotes lipid peroxidation.…”

“…It has recently been reported that exposure of the outer segments of rod photoreceptor cells, isolated from the frog retina, to bright light of 485 nm rapidly induces oxidative reactions followed by irreversible cell damage [4]. The light-induced oxidative process depends on a transient state of activated rhodopsin which promotes lipid peroxidation.…”

“…Melatonin induces a larger fluorescence response during illumination, probably as a consequence of its antioxidant effect during diabetes, which would provide more oxidizable lipids. [Diabetologia (2002) of intracellularly preloaded dehydrorhodamine (DHR) 123 into fluorescent rhodamine (RHO) 123, and by the presence of high levels of thiobarbituric acid reactive substances [4,5].…”