Melanopsin is an opsin-like protein found in the retinal ganglion cells of mammals. Melanopsin is believed to be part of a secondary optical system that parallels that of the classic rod and cone system. This second system reports directly to the suprachiasmatic nucleus and is responsible for regulation of circadian rhythms. Melanopsin is believed to be the primary photopigment responsible for the regulation of these circadian rhythms, and Melanopsin knockout mice have been generated which demonstrate decreased capacity to entrain to light and dark cycles.
