A mouse-like retinal cone phenotype in the Syrian hamster: S opsin coexpressed with M opsin in a common cone photoreceptor.

Previous immunocytochemical, physiological, and molecular studies have reported that the Syrian hamster lacks a shortwave-sensitive (S) cone photopigment but retains circadian responses to ultraviolet (UV) light. Using opsin antibodies and a sensitive detection protocol, we here show that S opsin immunoreactivity colocalizes with M opsin immunoreactivity in a common type of cone photoreceptor. S opsin signal within individual cone outer segments is low and continuously decreases from the ventral to dorsal retina. Only double-labeled cones were found. During development, S opsin expression precedes that of M opsin, but there is no indication of transdifferentiation. Our results imply that in the Syrian hamster low levels of S opsin colocalize with M opsin in a common cone phenotype. We suggest that, similar to other murid rodents, the S pigment absorbs maximally in the UV range, and thus may contribute to mediating the circadian response of the Syrian hamster to UV light.