Morphological Characterization of Ipsilaterally and Contralaterally-Innervated Neurons in the Mouse Superior Colliculus

Abstract

The superior colliculus (SC) is a major retinorecipient nucleus that regulates visually driven behaviors. Recently, binocular vision and innervation of the SC by ipsilaterally projecting retinal ganglion cells (ipsi-RGCs) were demonstrated to be necessary for prey capture behavior in mice. To better understand this, we sought to define the neuronal populations innervated by contralateral- (contra-RGCs) and/or ipsi-RGCs. We injected trans-synaptic AAV1-Cre and AAV1-Flp intraocularly in separate eyes and Cre- and Flp-dependent reporters in the SC. Reporter signal was amplified, and cell-specific markers labeled by immunohistochemistry, followed by collection of confocal images on cleared tissue, 3D reconstruction, and quantification of reconstructed cells. Out of a total of 353 reconstructed neurons, we found that 40.6% were innervated solely by contra-RGCs, 39.1% solely by ipsi-RGCs, and 20.3% were innervated by both. For each population, we found cells distributed across the entire rostro-caudal and mediolateral axes of the SC. Additionally, ipsi-RGC-recipient cells displayed a significantly smaller soma than contra-RGC-recipient cells. We also analyzed their morphology based on topographic location in the SC. Ipsi-RGC-recipient neurons in the rostro-medial and rostro-central SC have a higher dendritic complexity and a larger overall dendritic length than contra-RGC-recipient cells. In contrast, ipsi-RGC-recipient neurons in the caudo-medial SC have considerably less complexity in their proximal dendrites. These data suggest that neurons solely innervated by contra- or ipsi-RGCs may be comprised of distinct neuronal subtypes and, potentially, subserve different functions.