Identification of calretinin-expressing retinal ganglion cells projecting to the mouse superior colliculus

In mice, retinal ganglion cells (RGCs), which consist of around 30 subtypes, exclusively transmit retinal information to the relevant brain systems through parallel visual pathways. The superior colliculus (SC) receives the vast majority of this information from several RGC subtypes. The objective of the current study is to identify the types of calretinin (CR)-expressing RGCs that project to the SC in mice. To label RGCs, we performed CR immunoreactivity in the mouse retina after injections of fluorescent dye, dextran into mouse SC. Subsequently, the neurons double-labeled for dextran and CR were iontophoretically injected with the lipophilic dye, DiI, to characterize the detailed morphological properties of these cells. The analysis of various morphological parameters, including dendritic arborization, dendritic field size and stratification, indicated that, of the ten different types of CR-expressing RGCs in the retina, the double-labeled cells consisted of at least eight types of RGCs that projected to the SC. These cells tended to have small-medium field sizes. However, except for dendritic field size, the cells did not exhibit consistent characteristics for the other morphometric parameters examined. The combination of a tracer and single-cell injections after immunohistochemistry for a particular molecule provided valuable data that confirmed the presence of distinct subtypes of RGCs within multiple-labeled RGCs that projected to specific brain regions.