Opsin immunocytochemical characterization of different types of photoreceptors in the frog pineal organ.

The pineal organ of the frog, Rana esculenta and R. temporaria, was studied by opsin immunocytochemistry using two polyclonal antibovine rhodopsin and the monoclonal antichicken opsin antibodies OS-2 (detecting blue and green pigments) and COS-1 (detecting green and red pigments). Four types of photoreceptor cells were distinguished. The large outer segments of the numerous electron-dense photoreceptor cells ("large pineal rods") were immunoreactive with the rhodopsin and OS-2 antibodies, but reacted weakly with antibody COS-1. Some electron-dense photoreceptors with smaller outer segments ("small pineal rods") were found that were strongly OS-2-immunoreactive but moderately rhodopsin-positive. The long outer segments of the oil droplet containing photoreceptors ("large pineal cones") were only immunoreactive with the COS-1 antibodies. The small electron-lucent photoreceptors ("small pineal cones") were immunonegative with all the opsin antisera used. These results confirm the presence of the opsin of a (green-sensitive) rhodopsin in the "large rod" photoreceptors. A blue-sensitive pigment is supposed to be present in the "small rod" photoreceptors, and a red-sensitive one in the oil droplet-containing "large cones". The opsin-immunonegative "small cone" is discussed to contain a (UV-blue?) photopigment that differs essentially in its antigenic sites from the other pigments. The presence of four types of photoreceptors equipped with the opsins of apparently different photopigments strengthens the view that the frog pineal organ is capable of measuring different ranges of the light spectrum.