S-cone connections of the diffuse bipolar cell type DB6 in macaque monkey retina

Previous studies of primate retinae have shown that diffuse bipolar (DB) cells contact all the cones in their dendritic field, suggesting there is no spectral selectivity in the functional input to DB cells. However, since short-wavelength sensitive (S) cones make up less than 10% of the total cone population, specialized connectivity with S-cones is difficult to detect. In the present study, the S-cone connectivity of a subtype of DB cells, the DB6 cell, was studied in macaque monkey retina. Pieces of macaque retina were processed with antibodies to CD15 to stain DB6 cells and antibodies to the S-cone opsin to identify S-cones. Immunoreactivity was visualized using immunoperoxidase or immunofluorescence. Some preparations were additionally processed with peanut agglutinin coupled to fluorescein to reveal medium- and long-wavelength sensitive (M/L) cones. The preparations were analyzed using conventional and deconvolution light microscopy. The majority of DB6 cells had one or two S-cones in their dendritic field and the majority of S-cones were located in the dendritic field of DB6 cells. On average, 80% of the S-cones and 81% of the M/L cones contacted DB6 cells. The average number of dendritic terminals at cone pedicles did not differ between the cone types. However, the total number of DB6 dendritic terminals receiving input from M/L-cone pedicles was about eight times higher than the total number of dendritic terminals at S-cone pedicles. In conclusion, DB6 cells make indiscriminate contact with all cone types, but receive their major input from M/L-cones and thus carry a "Yellow-ON" spectral signal.