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 co ne bipolar cell in a rhesus monkey retina is described. The dendrites of representative Golgi-stained cells of each of the diffuse cone bipo lar cell types DB1 and DB6 were serially sectioned for EM examination. Bipolar cells of the DB1 type have axons terminating in the outer hal f of the inner plexiform layer. The dendrites of the cell examined wer e postsynaptic to seven cones at 71 basal synapses; in addition, they had two ribbon synapses with one cone, and one with another. A DB6 typ e of bipolar cell has axons ending in the inner half of the inner plex iform layer. The dendrites of the cell examined received input from se ven cones at 30 ribbon synapses; in addition there were 13 basal junct ions distributed between five of the seven cones contacted. Two invagi nating midget bipolar cells were found to be postsynaptic at 25 and 26 ribbon synapses of cone pedicles containing 44 and 40 ribbons respect ively. These results combined with our previously published work, show that the position and number of synapses is characteristic for each c ategory of cell. Those bipolar cells (flat) making basal synapses have more sites of synaptic contact with the cones than those bipolar cell s (invaginating) with predominantly ribbon synaptic input. Over 95% of the cone junctions of the three types of diffuse bipolar cell, DB1, D B2 and DB3, are basal; and their axons always end in the a-(Off-) laye r of the inner plexiform layer. All three types of diffuse invaginatin g cone bipolar cell, DB4, DB5 & DB6, have axons terminating in the b-( On-) layer of the inner plexiform layer; their dendrites are predomina ntly postsynaptic as central elements invaginating at the cone triads. However, unlike invaginating midget bipolar cells, whose dendrites ar e exclusively postsynaptic at ribbon synapses, between 10% (DB5) and 4 0% (DB4 and DB6) of the cone input to diffuse invaginating bipolar cel ls is through basal junctions. These data are discussed in the context of recent work on the synapses between foveal cones and their bipolar cells.