Horizontal cell connections with short wavelength-sensitive cones in the retina: A comparison between New World and Old World primates

Abstract

Recent studies in the Old World macaque monkey have shown that the two horizontal cell types H1 and H2 differ with respect to their connections to short wavelength-sensitive (SWS) cones. We wanted to establish whether this pattern of connectivity is common to all primates. The connections of horizontal cells with SWS cones were studied in the retinas of two species of New World (marmoset and tamarin) and two species of Old World (orangutan and chimpanzee) primates by using a double-labelling technique. Horizontal cells were labelled with DiI and then photoconverted; SWS cones were labelled immunocytochemically. The marmoset shows a sex-linked polymorphism of colour vision: All males are dichromats, whereas most females are trichromats. In contrast, Old World primates are usually trichromats. Our results show that the horizontal cells of both New World and Old World primates have a comparable pattern of connectivity with SWS cones and thus indicate that the wiring of horizontal cells with SWS cones does not differ between dichromats and trichromats and is common to all primates. The H1 cells make no or only sparse contact with SWS cones. In marmoset, H1 cells have on average 0.8% of their dendritic terminals at SWS cones. The H2 cells contact all SWS cones in their dendritic field. In marmoset, H2 cells have on average 11.8% of their dendritic terminals at SWS cones. The axon of H2 cells contacts SWS cones but presumably also contacts other cones.