Do Subsets of Widefield Vertical Cells Exist in the Mouse Superior Colliculus?

Projections from widefield vertical (WFV) cells of the superior colliculus to the pulvinar nucleus form a conserved pathway for motion detection across mammalian species. While subtypes of tectopulvinar cells have been identified in the ground squirrel, it is unclear whether subtypes can be identified in mice. Using viral tracing, immunohistochemistry, and confocal and electron microscopy in Tac1, Ntsr1-GN209, and C57BL/6J mice, we attempted to characterize WFV cells based on their morphology, pulvinar projection patterns, expression of substance P, parvalbumin, or hyperpolarization-activated cation channels (HCN1), and their retinal innervation. We found that the Ntsr1-GN209 line labels a subset of tectopulvinar cells that lack parvalbumin but express HCN1. Furthermore, comparison of previous ultrastructural datasets revealed that Ntsr1-GN209 WFV cells innervate the pulvinar with synaptic terminals that are smaller than the overall population of tectopulvinar terminals. However, using retrograde, anterograde, and transsynaptic viral tracing, we found that in all three mouse lines, WFV cells could not be subdivided based on their dendritic arbors or projections to the lateral pulvinar (Pl) or caudal medial pulvinar (Pcm). We also found that a subset of WFV cells receive ipsilateral retinal input, but these cells did not differentially innervate the Pl or Pcm. Finally, we found that the retina provides one-third of the synaptic input to Ntsr1-GN209 WFV cells and primarily innervates their small distal dendrites. Together, our data indicate that although the Ntsr1-GN209 line labels a subset, the overall population of mouse WFV cells cannot be subdivided based on features identified in the ground squirrel.