Lateral contacts and interactions of horizontal cell dendrites in the retina of the larval tiger salamander

Abstract

Contacts of horizontal cell dendrites with processes of other 2nd order neurons were studied by EM in serial sections of the salamander retina. Intracellular recordings of the responses to light of horizontal and bipolar cells were used to investigate the significance of some of the morphological findings. Horizontal cell dendrites make close membrane appositions (gap junctions) with one another and are post-synaptic to bipolar cell dendrites at presumed chemical synapses. Horizontal cell dendrites are apparently not presynaptic to any other neuronal processes at the outer plexiform layer. The output connections of horizontal cell bodies are unknown. Apparently the bipolar cell input and the gap junctions between dendrites contribute, respectively, depolarizing and hyperpolarizing components to the responses of horizontal cell bodies to surround illumination. The facilitative effect of central illumination on the surround response of horizontal cell bodies may result, although perhaps only partly, from observed properties of the surround response of bipolar cells. Bipolar cells were presynaptic at the outer plexiform layer to horizontal cell dendrites and to other bipolar cells, horizontal cell axon terminals and certain processes belonging to an unidentified neuron.