This study describes the connectivity between horizontal cells and short-wavelength-sensitive (SWS) cones in macaque monkey retina. H1 and H2 horizontal cells were either labelled with the carbocyanine dye, Dil, or injected intracellularly with Neurobiotin. The retinas were then processed with an antiserum against human SWS cone pigment, which usually stained the entire SWS cone. In these double-labelled retinas, the pattern of connectivity of H1 (n = 91) and H2 (n = 7) cells with SWS cones has been determined. About 85% of the H1 cells examined do not contact SWS cones. The dendritic terminal knobs of five H1 cells that do contact SWS cones were counted. They have, at most, 3% of their dendritic terminal knobs at SWS cones. All H2 cells examined make contact with SWS cones. The dendritic terminal knobs of one H2 cell were counted; about 11% of the dendritic terminal knobs are at the SWS cone. We conclude that horizontal cells in macaque monkey retina show specific patterns of connectivity to SWS cones.
