Rod photoreceptors signal fast changes in daylight levels using a CX36-independent retinal pathway in mouse

Temporal contrast detected by rod photoreceptors is channeled into multiple retinal rod pathways that ultimately connect to cone photoreceptor pathways via Cx36 gap junctions or via chemical synapses. However, we do not yet understand how the different rod pathways contribute to the perception of temporal contrast (changes in luminance with time) at mesopic light levels, where both rods and cones actively respond to light. Here, we use a forced-choice, operant behavior assay to investigate rod-driven, temporal contrast sensitivity (TCS) in mice of either sex. Transgenic mice with desensitized cones (GNAT2^cpfl3 line) were used to identify rod contributions to TCS in mesopic lights. We found that at low mesopic lights (400 photons/s/μm ² at the retina), control and GNAT2^cpfl3 mice had similar TCS. Surprisingly, at upper mesopic lights (8000 photons/s/μm ²), GNAT2^cpfl3 mice exhibited a relative reduction in TCS to low (<12 Hz) while maintaining normal TCS to high (12–36 Hz) temporal frequencies. The rod-driven responses to high temporal frequencies developed gradually over time (>30 min). Furthermore, the TCS of GNAT2^cpfl3 and GNAT2^cpfl3::Cx36 ⁻ /⁻ mice matched closely, indicating that transmission of high-frequency signals (1) does not require the rod-cone Cx36 gap junctions as has been proposed in the past; and (2) a Cx36-independent rod pathway(s) (e.g., direct rod to OFF cone bipolar cell synapses and/or glycinergic synapses from AII amacrine cells to OFF ganglion cells) is sufficient for fast, mesopic rod-driven vision. These findings extend our understanding of the link between visual circuits and perception in mouse.