M1 Intrinsically Photosensitive Retinal Ganglion Cells Integrate Rod and Melanopsin Inputs to Signal in Low Light

Light influences various behaviors and physiological processes that occur outside of our conscious perception, including circadian photoentrainment, sleep, and even learning and mood. The M1, melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) relay a combination of rod/cone and melanopsin signals to drive these functions. However, little is known about how M1 ipRGCs integrate these signals in low light. We measure the dim light response of M1 ipRGCs and find that they exhibit a wide spectrum of responses to dim, scotopic light stimulation that are driven by a combination of rod pathway input and melanopsin phototransduction. The presence of rod input to M1 ipRGCs correlates with larger and more complex dendritic arbors. Collectively, these results show variability in the rod input to M1 ipRGCs and a surprising contribution of melanopsin to the light responses of M1 ipRGCs at very low light.