Light drives the developmental progression of outer retinal function.

The complex nature of rod and cone photoreceptors and the light-evoked responsivity of bipolar cells in the mature rodent retina have been well characterized. However, little is known about the emergent light-evoked response properties of the mouse retina and the role light plays in shaping these emergent responses. We have previously demonstrated that the outer retina is responsive to green light as early as postnatal day 8 (P8). Here, we characterize the progression of both photoreceptors (rods and cones) and bipolar cell responses during development and into adulthood using ex vivo electroretinogram recordings. Our data show that the majority of photoreceptor response at P8 originates from cones and that these outputs drive second-order bipolar cell responses as early as P9. We find that the magnitude of the photoresponse increases concurrently with each passing day of postnatal development and that many functional properties of these responses, as well as the relative rod/cone contributions to the total light-evoked response, are age dependent. We compare these responses at eye opening and maturity to age-matched animals raised in darkness and found that the absence of light diminishes emergent and mature cone-to-bipolar cell signaling. Furthermore, we found cone-evoked responses to be significantly slower in dark-reared retinas. Together, this work characterizes the developmental photoresponsivity of the mouse retina while highlighting the importance of properly timed sensory input for the maturation of the first visual system synapse.