Excitatory dyad synapse in rabbit retina.

In the inner plexiform layer of the rabbit retina, the synaptic endings of bipolar cells contact a pair of postsynaptic processes at an unusual type of specialized junction, the dyad synapse. One of the members of the postsynaptic dyad may return conventional feedback synapses onto the bipolar endings. Freeze-fracturing demonstrates that, opposite the presynaptic active zone, both postsynaptic membranes contain an aggregate of intramembrane particles that remain associated with the outer leaflet (E face) of the fractured plasmalemma; this is a feature typical of excitatory synapses in the central nervous system. Intracellular recordings followed by injection of horseradish peroxidase showed that at the dyad synapse the endings of rod bipolar cells are usually presynaptic to the dendrites of two amacrine cells, one narrow-field and bistratified (AII) and the other wide-field (A17). Only the A17 rod amacrine cell returns feedback synapses onto the bipolar endings. Both amacrine cells respond to illumination with transient-sustained depolarizations, dominated by rods; thus, the polarity of their light responses is the same as that of rod bipolar cells. We conclude that the dyad synapses established by rod bipolar cells with the two types of amacrine cells are excitatory.