Encephalopsin

Encephalopsin, formally encoded by the OPN5 gene, is a member of the vertebrate opsin family of G protein-coupled receptors that binds retinal to form a light-responsive pigment. It is categorized as a non-visual or extra-retinal opsin because it is enriched in brain and other non-ocular tissues rather than in the classical retinal photoreceptors of the eye.

Expression studies across mammals and other vertebrates have detected OPN5 transcripts and protein in multiple brain regions, including the cerebral cortex and hypothalamus, as well as in the pineal gland and occasionally in peripheral tissues. The distribution can vary by species, developmental stage, and method of detection.

Functionally, encephalopsin is thought to participate in photic signaling outside the retina, contributing to non-image-forming light responses, such as circadian or hormonal regulation. However, precise in vivo roles and signaling pathways are not fully established. In vitro analyses show that OPN5 can form a light-sensitive pigment and activate G protein signaling in a manner typical of opsins, though the coupling preferences and physiological relevance remain under study.

Evolutionarily, OPN5 is conserved across vertebrates, suggesting an ancient, widespread role for brain- or non-ocular photo-detection. The identification of encephalopsin expanded the understanding of how light information may influence physiology beyond the traditional retinal pathways.