Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN

The recent discovery of inner retinal photoreceptors in mammals and fish was a major surprise. Present in addition to the well known rods and cones, these receptors are thought to detect irradiance levels, and to be linked to the night-and-day regulation of the circadian system. Two new studies show that melanopsin, found almost exclusively in these ‘ganglion-cell photoreceptors’, is photosensitive. Qiu et al. turn mammalian kidney cells into functional photoreceptors by introducing melanopsin, and Melyan et al. do a similar trick in neuronal cells. These findings could have clinical applications, possibly allowing selective stimulation of cells in the brain and helping to restore sight lost due to retinal degeneration. A further study identifies a previously unknown retinal population of ‘giant’ melanopsin-expressing ganglion cells. They are photosensitive but are also activated by rods and cones, thereby merging the conventional ‘image forming’ pathway with the radiance-detecting pathway in primates.