Slow vision: Measuring melanopsin-mediated light effects in animal models

Mammalian retinas contain three specialized photoreceptors: the rods and cones in the outer retina, whose primary function is to support visual perception in dim and bright environments, respectively, and a small subset of retinal ganglion cells ("intrinsically photosensitive" retinal ganglion cells; ipRGCs), which are directly light-responsive owing to their expression of the photopigment melanopsin. Melanopsin photoreception is optimized to encode low-frequency changes in the light environment and, as a result, extends the temporal and spatial range over which light is detected by the retina. ipRGCs innervate many brain areas, and this allows melanopsin light responses to be used for diverse purposes, ranging from the synchronization of the circadian clock with the solar day to light's regulation of mood, alertness, and neuroendocrine and cognitive functions. In this review, we discuss the methods and findings that have contributed to our understanding of melanopsin across biology. We particularly focus on the approaches that allow melanopsin to be studied at a systems/whole animal level and how these methods have illuminated the role of melanopsin in diverse physiological outputs.