A novel human opsin in the inner retina

Here we report the identification of a novel human opsin, melanopsin, that is expressed in cells of the mammalian inner retina. The human melanopsin g ene consists of 10 exons and is mapped to chromosome 10q22. This chromosoma l localization and gene structure differs significantly from that of other human opsins that typically have four to seven exons. A survey of 26 anatom ical sites indicates that, in humans, melanopsin is expressed only in the e ye. In situ hybridization histochemistry shows that melanopsin expression i s restricted to cells within the ganglion and amacrine cell layers of the p rimate and murine retinas. Notably, expression is not observed in retinal p hotoreceptor cells, the opsin-containing cells of the outer retina that ini tiate vision. The unique inner retinal localization of melanopsin suggests that it is not involved in image formation but rather may mediate nonvisual photoreceptive tasks, such as the regulation of circadian rhythms and the acute suppression of pineal melatonin. The anatomical distribution of melan opsin-positive retinal cells is similar to the pattern of cells known to pr oject from the retina to the suprachiasmatic nuclei of the hypothalamus, a primary circadian pacemaker.