Photoreceptor degeneration in vitamin A deprivation and retinitis pigmentosa: the equivalent light hypothesis.

Long-term exposure of the retina to constant illumination is known to produce irreversible degeneration of photoreceptors. We propose that similar mechanisms may be involved in photoreceptor degeneration produced by vitamin A deprivation and some forms of retinitis pigmentosa (RP). Evidence is reviewed suggesting that the free opsin present during vitamin A deprivation or the mutated opsin present in some forms of RP excite the visual transduction cascade. This would produce a constant 'equivalent light' that triggers photoreceptor degeneration. Continuous real or equivalent light may produce outer segment degeneration by interfering with circadian processes, such as protein synthesis and disc shedding and lead to the loss of photoreceptors including those not expressing the mutant gene.