Cyclic AMP as a Regulator of the Phototransduction Cascade

Abstract

Until recently, cAMP has been believed to play an important role in the organization of circadian rhythms in the vertebrate eye but not to be directly involved in controlling the phototransduction cascade in photoreceptors. However, given that cAMP levels in photoreceptor cells vary significantly during the day and that cAMP metabolism in photoreceptor cells may itself be light-dependent, it is logical to suggest that cAMP-dependent protein phosphorylation may be a mechanism for tuning phototransduction processes in accordance with ongoing illumination. The current review addresses contemporary data on the structure of the circadian pacemaker in the retina, the involvement of intraphotoreceptor cAMP in its organization, and the known intracellular targets on which cAMP has or may have regulatory influences. Our recent data indicate that changes in cAMP levels have regulatory influences on the photoreceptor cascade. This action may significantly widen the range of adaptation of photoreceptors to illumination conditions, increasing its sensitivity during the dark part of the day and decreasing its sensitivity in bright light.