Non-image-forming functional roles of OPN3, OPN4 and OPN5 photopigments

Detecting different wavelengths and intensities of environmental light is crucial for the survival of many animals. In response, a multiplicity of opsins (a special group of photosensitive G protein-coupled receptors), when combined with a retinal chromophore, is able to directly detect light and initiate different downstream phototransduction signaling cascades. Although avian studies from the 1930s suggested the presence of deep brain photoreceptors that could respond to seasonal changes in the light/dark cycle, it was only a few decades ago that photopigments other than those found in the visual system (i.e. rods and cones) were identified as functional photoreceptors. It is now established that several classes of non-visual photoreceptors and the photopigments they express, in lower vertebrates to higher mammals alike, can regulate a plethora of mechanisms that function outside of vision. These include the synchronization of light/dark cycles with biological/cellular rhythms of the body (i.e. photoentrainment); melanogenesis in dermal tissues; thermoregulation in adipose tissue; embryonic eye development; smooth muscle relaxation; and the development of certain cancers. These and other mechanisms have been shown, in part at least, to be controlled by the expression of three important non-visual opsin genes, namely OPN3, OPN4 and OPN5, although other vertebrate opsin classes exist, many with unknown or unclear functional roles assigned to them presently. Specifically, these three opsins have been shown to be expressed during early embryogenesis and throughout adulthood, which will be discussed here. Moreover, this review highlights recent studies that focus on several key non-image-forming functional roles of OPN3, OPN4 and OPN5, and in particular those that impact photoreception in developing structures and pathways, as well as in adulthood.