Specialized synaptic pathway for chromatic signals beneath S-cone photoreceptors is common to human, Old and New World primates.

The distribution of the soluble NSF-attachment protein receptor protein syntaxin-4 and the Na-K-Cl cotransporter (NKCC) were investigated in the outer plexiform layer of human retina using immunohistochemistry. Both proteins, which are proposed to be components of a gamma-aminobutyric acid mediated feed-forward circuit from horizontal cells directly to bipolar cells, were enriched beneath S-cones. The expression pattern of syntaxin-4 was further analyzed in baboon and marmoset to determine if the synaptic specialization is common to primates. Syntaxin-4 was enriched beneath S-cones in both species, which together with the human results indicates that this specialization may have evolved for the purpose of mediating unique color vision capacities that are exclusive to primates.