Retinal ganglion cells expressing melanopsin.

The daily alternation of light and darkness resulting from Earth's rotation is one of the main environmental cues shaping the internal life of animals, orchestrating rhythms of behavior and physiologic functions. In mammals, the retina, lining the back of the eye, serves as the sole entry point for this photic information. About 25 years ago, the discovery of a novel photopigment revolutionized our understanding of how light influences the brain and body. This photopigment, highly conserved across evolution, relies on a protein named melanopsin. Research on melanopsin has revealed the presence of an intrinsically photosensitive system within the retinal ganglion cells that operates independently, at least in part, from the classic visual pathways. This chapter aims to provide a comprehensible overview of melanopsin function in the mammalian retina. We will start by recalling the context of its discovery, examine its expression patterns in the retina, explore its broad implications in physiologic functions, and, finally, discuss its significance in various diseases.