Rod photoreceptors drive circadian photoentrainment across a wide range of light intensities

In mammals, synchronization of the circadian pacemaker in the hypothalamus is achieved through direct input from the eyes conveyed by intrinsically photosensitive retinal ganglion cells (ipRGCs). Circadian photoentrainment can be maintained by rod and cone photoreceptors, but their functional contributions and their retinal circuits that impinge on ipRGCs are not well understood. We demonstrate in genetic mouse models lacking functional rods, or where rods are the only functional photoreceptors, that rods are solely responsible for photoentrainment at scotopic light intensities. Surprisingly, rods were also capable of driving circadian photoentrainment at photopic intensities where they were incapable of supporting a visually–guided behavior. Using animals in which cone photoreceptors were ablated, we demonstrate that rods signal through cones at high light intensities, but not low light intensities. Thus two distinct retinal circuits drive ipRGC function to support circadian photoentrainment across a wide range of light intensities.