Bleaching adaptation in photoreceptors.

Bleaching adaptation is the reduction in sensitivity of photoreceptors to light, following large photoconversion of rhodopsin molecules. It is accompanied by a prolonged dark excitation, and the two processes may be causally linked. There are remarkable similarities in the prolonged dark excitation of vertebrate rods and invertebrate photoreceptors. There is much evidence to indicate that prolonged dark excitation is tightly linked to processes that occur at the photo-pigment level. A molecular mechanism is suggested to explain the prolonged dark excitation. According to this model, quenching of photoreceptor excitation operates by phosphorylation of metarhodopsin (M) molecules. When unphosphorylated M molecules exist in the dark, they give rise to prolonged dark excitation. This prolonged dark excitation seems to be coupled to photoreceptors' desensitization by an unknown mechanism.