A non-canonical photopigment, melanopsin, is expressed in the differentiating ganglion, horizontal, and bipolar cells of the chicken retina

Vertebrate melanopsin is a photopigment in the eye, required for photoentrainment. Melanopsin is more closely related to opsin proteins found in invertebrates, than to the other photo-pigments. Although the invertebrate melanopsin-like protein is localized in rhabdomeric photoreceptors in the invertebrate eye, it has been shown to be expressed in a subset of retinal ganglion cells in the mouse and in horizontal cells in the frog, indicating its diversified expression pattern in vertebrates. Here we show that two types of melanopsin transcripts are expressed in the developing chicken retina. Melanopsin is firstly expressed by a small subset of ganglion cells, and then prominently expressed by horizontal cells and later by bipolar cells in the developing chicken retina. This suggests that a subset of ganglion, horizontal, and bipolar cells in the chicken retina may have rhabdomeric properties in their origins.