Non-visual opsins and their role in circadian photoentrainment

Photoreception is common in animals without a visual system. In animals with visual systems, it is sometimes presumed that the same photoreceptors and pathways will accommodate both visual and non-visual light detection. However, mounting evidence reveals that most animals exhibit broad extra-visual photoreceptive functions that are wholly independent of the visual system. One of these functions is the synchronization of the circadian clock to light-dark signals, or photoentrainment. In mammals, behavioral photoentrainment is achieved exclusively through visual and non-visual opsin proteins within the retina, and molecular photoentrainment of individual cells occurs using non-visual opsins in some peripheral tissues. This is in contrast to insects and fish where nearly all peripheral organs are directly photoentrainable. This review will summarize the family of opsins in mammals and focus on the role of non-visual opsins in circadian photoreception. Particular emphasis will be given to photoentrainment in other vertebrates in order to compare and contrast the use of the wide range of non-visual opsins in circadian photoentrainment throughout the animal kingdom.