Non-rod, non-cone photoreception in rodents and teleost fish

Until recently, all ocular photoreception was attributed to the rods and cones of the retina. However, studies on mice lacking rod and cone photoreceptors (rd/rd cl), has shown that these mice can still use their eyes to detect light to regulate their circadian rhythms, suppress pineal melatonin, modify locomotor activity and modulate pupil size. In addition, action spectra for some of these responses have characterized a novel opsin/vitamin A-based photopigment with a λmax ∼480 nm. Electrophysiological studies have shown that a subset of retinal ganglion cells are intrinsically photosensitive, and melanopsin has been proposed as the photopigment mediating these responses to light. In contrast to mammals, an inner retinal photopigment gene has been identified in teleost fish. Vertebrate ancient (VA) opsin forms a photopigment with a λmax between 460-500 nm, and is expressed in a sub-set of retinal horizontal cells, and cells in the amacrine and ganglion cell layers. Electrophysiological analysis suggests that VA opsin horizontal cells are intrinsically photosensitive and encode irradiance information. In contrast to mammals, however, the function of these novel ocular photoreceptors remains unknown. We compare non-rod, non-cone ocular photoreceptors in mammals and fish, and examine the criteria used to place candidate photopigment molecules into a functional context.