Spatial Distribution and Morphology of CaMKII-Expressing Amacrine Cells in Marmoset, Macaque, and Human Retina

Over 30 types of amacrine cells have been described in the primate retina, yet few are well characterized. Here, we investigated amacrine cells expressing the alpha subunit of calcium/calmodulin-dependent protein kinase II (CaMKII) in the retinas of human, macaque (Macaca fascicularis, Macaca nemestrina), and marmoset (Callithrix jacchus) monkeys using immunohistochemistry and intracellular injections, with a focus on displaced amacrine cells (dACs) in the ganglion cell layer. The spatial density of CaMKII-positive dACs decreases with the distance from the fovea, but in the peripheral temporal retina, the density of CaMKII-positive dACs nevertheless exceeds the density of retinal ganglion cells. In all species, CaMKII-positive dACs include cells expressing choline acetyltransferase (ChAT) cells, but in the human retina, only 60% of the ON ChAT population is CaMKII-positive. Conversely, in the marmoset and the macaque, about 80% of ON ChAT cells co-express CaMKII, but only 55% of ON ChAT cells in humans do so. Intracellular injections of CaMKII-positive dACs with the lipophilic dye DiI revealed ON starburst and semilunar Type 3 cells in all three species, but in the human retina, at least three additional types were detected. In the inner nuclear layer, CaMKII is expressed by multiple populations of amacrine cells, which are distinguished based on their soma size and staining intensity, but OFF ChAT cells do not co-express CaMKII. We conclude that ON- and OFF-ChAT cells show distinct patterns of CaMKII expression and that the diversity of CaMKII-expressing dACs in humans is greater than that in marmoset or macaque retina.