Gap junctions between AII amacrine cells and calbindin-positive bipolar cells in the rabbit retina

Electrical synapses or gap junctions occur between many retinal neurons. Ho wever, in most cases, the gap junctions have not been visualized directly. Instead, their presence has been inferred from tracer spread throughout the network of cells. Thus, tracer coupling is taken as a marker for the prese nce of gap junctions between coupled cells. AII amacrine cells are critical interneurons in the rod pathway of the mammalian retina. Rod bipolar cell output passes to AII amacrine cells, which in turn make conventional synaps es with OFF cone bipolar cells and gap junctions with ON cone bipolar cells . Injections of biotinylated tracers into AII amacrine cells reveals coupli ng between the AII amacrine cell network and heterologous coupling with a v ariety of ON cone bipolar cells, including the calbindin-positive cone bipo lar cell. To directly visualize gap junctions in this network, we prepared material for electron microscopy that was double labeled with antibodies to calretinin and calbindin to label AU, amacrine cells and calbindin-positiv e cone bipolar cells, respectively. AIT amacrine cells were postsynaptic to large vesicle-laden rod bipolar terminals, as previously reported. Gap jun ctions were identified between AII amacrine cells and calbindin-positive co ne bipolar cell terminals identified by the presence of immunostaining and ribbon synapses. This represents direct confirmation of gap junctions betwe en two different yet positively identified cells, which are tracer coupled, and provides additional evidence that tracer coupling with Neurobiotin ind icates the presence of gap junctions. These results also definitively estab lish the presence of gap junctions between AII amacrine cells and calbindin bipolar cells which can therefore carry rod signals to the ON alpha gangli on cell.