Intrinsically photosensitive retinal ganglion cells

Background: Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGC) area third class of photoreceptors in the retina in addition to rods and cones. They are a small heterogeneous population of cells primarily mediating non-image-forming visual functions. Objective: This article provides an overview of the current understanding of the functions and the diversity of ipRGCs. It moreover gives an insight into clinically and translationally relevant aspects and treatment options. Material and methods Narrative review article. Results: ipRGCs make up similar to 1-2% of all retinal ganglion cells and are divided into 6 specialized subtypes. With the photopigment melanopsin they can trigger light responses without rod or cone input and can relay irradiance information to various centers of the brain. Depending on the subtype, ipRGCs mediate non-image-forming tasks, such as the pupillary light reflex or synchronizing the circadian clock, and image-forming tasks, such as contrast optimization. ipRGCs exhibit differential resilience against optic nerve damage, making them an interesting study object for the development of neuroprotective strategies. In addition, melanopsin is an attractive optogenetic tool for vision restoration. Conclusion: Knowledge on ipRGC physiology is indispensable for understanding frequent clinical observations. Their functional and morphological features are the subject of active research, which highlights novel translational strategies.