# Non-visual opsins and their role in circadian photoentrainment

## TL;DR

This paper reviews non-visual opsins and their role in syncing body clocks to light-dark cycles across different animals.

## Contribution

The paper provides a comparative overview of non-visual opsins in circadian photoentrainment across vertebrates.

## Key findings

- Mammals use both visual and non-visual opsins in the retina for circadian photoentrainment.
- Non-visual opsins in peripheral tissues can also synchronize cellular clocks in mammals.
- Insects and fish show direct photoentrainment in nearly all peripheral organs.

## Abstract

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.