Rod/cone coupling is the entry point to the secondary rod pathway. Our overall hypothesis is that the circadian and light-induced modulation of rod/cone coupling changes retinal function and has profound effects throughout the visual system according to the time of day. Electrical synapses, also known as gap junctions, are common building blocks that connect neurons into coupled networks. Although electrical synapses display a high degree of plasticity, there is a fundamental gap in understanding how this plasticity modifies circuit activity and output. We anticipate that learning how to control electrical coupling may have useful clinical potential. We have developed (1) the capability to record from pairs of adjacent mouse photoreceptors to directly measure the trans-junctional conductance; (2) rod-specific and cone-specific connexin36 (Cx36) knockout (XO) mouse lines. In these mice, there is no rod/cone coupling, mimicking daytime or bright light. (3) a phospho-mimetic mutant Cx36 conditional knock-in (Cx36-DEDD) line, which displays saturated rod/cone coupling, equivalent to night time; and (4) a congenic B6 mouse line in which we rescued melatonin synthesis—an important circadian clock signal that is missing in most mouse strains. Retinas from the congenic line show robust circadian variations in dopamine release. In aim 1, by recording from rod/cone pairs, we will measure the gap junction conductance between rods and cones to test the hypothesis that rod/cone coupling spans from ~ 0 to 1,000+ pS, reflecting the collective action of melatonin, dopamine, and ambient light. We have shown there is no rod/cone coupling in the Cx36 XOs (mimics daytime) and we expect maximal coupling in the Cx36-DEDD (mimics nighttime). We will determine how the rod/cone gap junction conductance changes by time of day (congenic B6 line). In aim 2, we will record from cones and from ganglion cells to test the hypothesis that rod signals in the secondary rod pathway change by time of day. We expect our mutant lines to set the limits, with minimal input in the Cx36 XOs (mimics daytime) and maximal input in the Cx36-DEDD line (mimics nighttime). In aim 3, we will examine visual behavior in the intact mouse to measure the effect of an inactive (Cx36 XO, mimics daytime) and of a saturated rod/cone pathway (Cx36-DEDD, mimics nighttime). We will test the hypothesis that rod/cone coupling plasticity contributes to the daily modulation of contrast sensitivity and visually- guided behavior. Our work will offer a prime example of how a single electrical synapse can change retinal function to influence visual perception. This research will help define general principles underlying the role of circadian clocks and electrical synaptic plasticity in the daily changes in neural circuits relevant to brain function and behavior.