opn4

OPN4, commonly referred to as opsin-4 or melanopsin, is a gene encoding a photopigment expressed in intrinsically photosensitive retinal ganglion cells (ipRGCs) of many vertebrates. The encoded protein is a light-sensitive G-protein-coupled receptor that uses 11-cis-retinal as a chromophore to detect blue light, with peak sensitivity around 480 nm. In ipRGCs, melanopsin contributes to non-image-forming visual responses rather than forming detailed visual images.

Melanopsin is a seven-transmembrane domain receptor that activates a Gq/11 signaling cascade upon photon absorption, leading to phospholipase C activation, increased intracellular calcium, and neuronal depolarization. The signal from these cells is conveyed to brain regions such as the suprachiasmatic nucleus (SCN), which coordinates circadian rhythms, and the pretectal area, which influences the pupil light reflex.

Expression patterns and functional roles of OPN4 have made it a central component of non-image-forming photoreception. In humans and other mammals, melanopsin-containing ipRGCs help regulate circadian phase, alertness, melatonin suppression, and pupil constriction in response to ambient light. The gene shows evolutionary conservation across vertebrates, reflecting its fundamental role in adapting physiology and behavior to daily light cycles. Some species express multiple melanopsin isoforms through alternative splicing, which may modulate spectral sensitivity or signaling properties.

In research and clinical contexts, OPN4 serves as a marker for ipRGCs and is studied for its contributions to sleep disorders, seasonal biology, and light-based therapies that aim to influence circadian and mood-related processes.