Synaptic organization of the cone horizontal cells in the catfish retina.

Horizontal cells of the vertebrate retina are known to contribute to the formation of the receptive field surrounds of photoreceptor and bipolar cells. However, few synapses have been described anatomically that might mediate these interactions. We have observed in the catfish retina that cone horizontal cell perikarya and dendrites make conventional chemical synapses onto photoreceptor terminal telodendria and onto bipolar cell dendrites, while horizontal cell axon terminals make chemical synapses onto the perikarya and processes of amacrine cells. The synapses are characterized by clusters of round vesicles aggregated close to the site of contact, as well as by electron-dense material associated with both pre- and postsynaptic membranes. The three kinds of synapses observed anatomically correspond to the synaptic pathways involving cone horizontal cells that have been suggested by the physiology of these cells.