Morphological identification of ganglion cells expressing the α subunit of type II calmodulin-dependent protein kinase in the macaque retina

Expression of the α subunit of type II calmodulin-dependent protein kinase (αCamKII) distinguishes the koniocellular neurons of the primate lateral geniculate nucleus (LGN) from the primary parvo- and magnocellular neurons, but whether the same neurochemical signature distinguishes the retinal ganglion cells providing them input is not known. We find that, in the retina, αCamKII expression also differentiates two primary groups of ganglion cell, both characterized by broad, sparsely branching dendritic trees and cell bodies intermediate in size between the parvo- and magnocellular-projecting ganglion cells. Cells in the first group have three or four primary dendrites, a thick axon, and a rounded cell body and likely are made up of multiple types. In contrast, ganglion cells in the second group demonstrate a highly regular morphology, with strictly two primary dendrites and a thinner axon emanating from a smaller, elliptical cell body. This cell resembles the "large sparse" ganglion cell identified by others in retrograde labeling from the LGN and represents about 2% of all ganglion cells. In the optic nerve, αCamKII+ axons are also intermediate in size and form a bimodal distribution, correlating with the axonal sizes of the two groups of ganglion cell. For the LGN, we describe a group of αCamKII+ axon terminals with morphology consistent with terminals from retinal ganglion cells. These terminals form long, filamentous contacts with αCamKII+ relay cells and increase in frequency from the dorsal to the ventral koniocellular regions. Our results indicate that ganglion cells expressing αCamKII represent multiple projections to the brain, at least one of which provides input to one or more koniocellular regions of the LGN.