Melanopsin cells are the principal conduits for rod–cone input to non-image-forming vision

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Abstract
The mammalian retina has three types of light-sensing cells: rods, cones and melanopsin-containing cells. Rods and cones are involved in vision but have also been shown to contribute to light entrainment of the circadian clock. Now Güler et al. show that the non-image forming (circadian) role of rods and cones involves signalling via melanopsin-containing cells. This finding implies that people with troubled sleep or seasonal depression could benefit from light detection and melatonin suppression tests even if they are normally sighted. The mammalian retina has three types of light-sensing cells: rods, cones and melanopsin-containing cells. Rods and cones are involved in vision but have also been shown to contribute to light-entrainment of the circadian clock. Rods and cones must signal through melanopsin-containing cells for the latter. Rod and cone photoreceptors detect light and relay this information through a multisynaptic pathway to the brain by means of retinal ganglion cells (RGCs)1. These retinal outputs support not only pattern vision but also non-image-forming (NIF) functions, which include circadian photoentrainment and pupillary light reflex (PLR). In mammals, NIF functions are mediated by rods, cones and the melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs)2,3. Rod–cone photoreceptors and ipRGCs are complementary in signalling light intensity for NIF functions4,5,6,7,8,9,10,11,12. The ipRGCs, in addition to being directly photosensitive, also receive synaptic input from rod–cone networks13,14. To determine how the ipRGCs relay rod–cone light information for both image-forming and non-image-forming functions, we genetically ablated ipRGCs in mice. Here we show that animals lacking ipRGCs retain pattern vision but have deficits in both PLR and circadian photoentrainment that are more extensive than those observed in melanopsin knockouts8,10,11. The defects in PLR and photoentrainment resemble those observed in animals that lack phototransduction in all three photoreceptor classes6. These results indicate that light signals for irradiance detection are dissociated from pattern vision at the retinal ganglion cell level, and animals that cannot detect light for NIF functions are still capable of image formation.