Molecular specificity of defined types of amacrine synapse in cat retina.

The inner plexiform layer of cat retina contains synaptic structures belonging to 50 or more types of "identified" neurons. To learn whether there are antigens confined to subsets of these synaptic structures, we raised monoclonal antibodies to homogenates of neural retina. Binding patterns of these antibodies were visualized by the peroxidase-antiperoxidase method and studied in serial, ultrathin sections by electron microscopy. Four antibodies stained the synaptic varicosities of certain amacrine cells. Many of the stained varicosities formed reciprocal synapses with a rod bipolar axon terminal, but only about half of the reciprocal synapses associated with a rod bipolar were stained. Other stained varicosities formed synapses with cone bipolar axons, ganglion cell dendrites, and unstained amacrine processes. The patterns were essentially the same for each antibody and were not altered by staining with the antibodies two at a time; therefore, it is likely that all four antibodies stain the same subset of synaptic structures. These patterns would be accounted for if there were staining of all the synaptic varicosities of three of the four types of identified amacrine reciprocally connected to the rod bipolar (A6, A8, A13). This localization suggests that the antigen responsible for the binding pattern is not associated with synaptic transmission. Staining is present in the inner plexiform layer during the period of synaptogenesis and consequently the antibodies are serving as markers for following the development of identified synapses in an identified neural circuit.