Ocular and extraocular roles of neuropsin in vertebrates

The ability to detect and adapt to different levels of ambient light is critical for animal survival. Light detection is the basis of vision, but light also regulates eye development and drives several non-image-forming functions, including synchronizing circadian rhythms to the daily light/dark cycle, restricting pupils in response to changes in light intensity, and modulating mood in response to light. Until the early 2000s, these functions were thought to be solely mediated by ocular photoreceptors. However, neuropsin (OPN5), a UV-sensitive opsin, has been receiving growing attention, as new methods have revealed previously unappreciated functions of OPN5. In fact, OPN5-mediated extraocular and deep-brain photoreception have recently been described for the first time in mammals. This review aims to synthesize current knowledge of the properties and functions of OPN5 across vertebrates. Several aspects of vertebrate physiology are regulated by light. Until recently, it was thought that light perception was mediated solely by retinal photoreceptors in mammals (rods, cones, and melanopsin cells), and by retinal and pineal opsin (pinopsin) in birds.An additional opsin, neuropsin (OPN5), sensitive to UV and short-wavelength light, has been detected in multiple species of vertebrates, including birds, rodents, and humans. This opsin is expressed in both neuronal (retina and brain) and epithelial tissues (cornea and skin).This new opsin, OPN5, has been proposed to regulate circadian functions (entrainment and acute response to light) and noncircadian functions (eye development, endocrine regulation) in mammals and nonmammals.The recent discovery of OPN5 functions reshapes current perspectives about the photoreception system of vertebrates by extending it to peripheral tissues, and could represent the first deep-brain photoreception observed in mammals.