Light Screen in Eyes

The stimulation of a visual receptor in a vertebrate eye by an intense flash of light generates a fast electrical response, the early receptor potential (RP). A similar response in the pigment epitheUum-choroid complex (PE-CC) of the eye was observed by Brown (26) which unlike the early RP is photostable and resistant to light adaptation. The PE-CC consists of the cell layers immediately behind the retina which are densely pigmented with melanin. Brown concluded that the photopigment involved in the PE-CC response was related to the visual pigments. 

One of the most characteristic properties of melanins is their ability to exist in both the oxidized quinone and reduced quinol forms by acting either as electron acceptors or electron donors in reaction with reducing and oxidizing agents respectively, thus exhibiting their dual functionality (33). These processes merely involve the reversible exchange of two electrons and two protons (33, 112, 138b). 

The electron-exchange properties of melanins have been studied with a number of especial reagents in order to understand the reaction mechanism as well as the role of melanin redox properties in biological systems. The processes have been found to be strongly irradiation dependent (both by visible and UV light). Thus, nitroxide radicals are reversibly reduced by melanins in the dark, and the redox equilibria are altered on irradiation (233). Similarly, other processes in living systems 

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