The cone synapses of DB1 diffuse, DB6 diffuse and invaginating midget, bipolar cells of a primate retina

The distribution of synapses between cones and two types of diffuse cone bipolar cell in a rhesus monkey retina is described. The dendrites of representative Golgi-stained cells of each of the diffuse cone bipolar cell types DB1 and DB6 were serially sectioned for EM examination. Bipolar cells of the DB1 type have axons terminating in the outer half of the inner plexiform layer. The dendrites of the cell examined were postsynaptic to seven cones at 71 basal synapses; in addition, they had two ribbon synapses with one cone, and one with another. A DB6 type of bipolar cell has axons ending in the inner half of the inner plexiform layer. The dendrites of the cell examined received input from seven cones at 30 ribbon synapses; in addition there were 13 basal junctions distributed between five of the seven cones contacted. Two invaginating midget bipolar cells were found to be postsynaptic at 25 and 26 ribbon synapses of cone pedicles containing 44 and 40 ribbons respectively. These results combined with our previously published work, show that the position and number of synapses is characteristic for each category of cell. Those bipolar cells (flat) making basal synapses have more sites of synaptic contact with the cones than those bipolar cells (invaginating) with predominantly ribbon synaptic input. Over 95% of the cone junctions of the three types of diffuse bipolar cell, DB1, DB2 and DB3, are basal; and their axons always end in the a- (Off-) layer of the inner plexiform layer. All three types of diffuse invaginating cone bipolar cell, DB4, DB5 & DB6, have axons terminating in the b- (On-) layer of the inner plexiform layer; their dendrites are predominantly postsynaptic as central elements invaginating at the cone triads. However, unlike invaginating midget bipolar cells, whose dendrites are exclusively postsynaptic at ribbon synapses, between 10% (DB5) and 40% (DB4 and DB6) of the cone input to diffuse invaginating bipolar cells is through basal junctions. These data are discussed in the context of recent work on the synapses between foveal cones and their bipolar cells.