SYNAPSES BETWEEN CONES AND DIFFUSE BIPOLAR CELLS OF A PRIMATE RETINA

The photoreceptor synapses of three representative cells of the six ty pes of diffuse bipolar cell of the rhesus macaque monkey's retina are described at 3.5-4.0 mm eccentricity. Bipolar cell DB3 was found to be postsynaptic to 11 cones at 155 basal synapses; about 70% of these we re triad associated. Bipolar cell DB4 was postsynaptic to eight cones at 52 ribbon synapses; in addition it was found also to make an averag e of two or three basal (non-ribbon) synapses per cone (total 23). The DB5 bipolar cell type had 57 invaginating synapses with seven cones. It too had basal synapses, but only two with each of three cones. The diffuse invaginating bipolar cell described by Mariani (1981) is ident ified as a member of the DB5 category. Dendrites of cone bipolar cell types which have axons ending in the a-layer of the inner plexiform la yer make only basal synapses with the cone pedicle. Those so far inves tigated are the flat midget bipolar cell and the DB2 and DB3 flat diff use bipolar cells. All bipolar cells whose axons terminate in the b-la yer of the inner plexiform layer. are postsynaptic at the ribbon synap ses of the cone pedicles. They now appear to fall into two groups. Tho se whose dendrites are exclusively postsynaptic at the ribbons; these are the blue cone and invaginating midget bipolar cells. And the diffu se bipolar cell DB4, that has both ribbon and basal synapses in a rati o of about 2.3:1. It is uncertain into which category cell DB5 should be placed; its basal synapses are so few the cell could be anomalous. It now seems that at least one primate bipolar cell type may be like t hose of other vertebrates in having, as defined ultrastructurally, two different kinds of synaptic connection with its cones. The results ar e discussed in the context of a brief review of the photoreceptor syna pses of other mammalian bipolar cells.