Simultaneous contribution of two rod pathways to AII amacrine and cone bipolar cell light responses

Rod signals traverse several synapses en route to cone bipolar cells. In one pathway, rods communicate directly with cones via gap junctions. In a second pathway, signals flow rods-rod bipolars-AII amacrines-cone bipolars. The relative contribution of each pathway to retinal function is not well understood. Here we have examined this question from the perspective of the All amacrine. AIIs form bidirectional electrical synapses with ON cone bipolars. Consequently, as ON cone bipolars are activated by outer plexiform inputs, they too should contribute to the All response. Rod bipolar inputs to AIIs were blocked by AMPA receptor antagonists, revealing a smaller, non-AMPA component of the light response. This small residual response did not reverse between -70 and +70 mV and was blocked by carbenoxolone, suggesting that the current arose in ON cone bipolars and was transmitted to AIIs via gap junctions. The residual component was evident for stimuli 2 log units below cone threshold and was prolonged for bright stimuli, demonstrating that it was rod driven. Because the rod bipolar-All pathway was blocked, the rod-driven residual current likely was generated via the rod-cone pathway activation of ON cone bipolars. Thus for a large range of intensities, rod signals reach the inner retina by both rod bipolar-All and rod-cone coupling pathways.