Morphological and Molecular Distinctions of Parallel Processing Streams Reveal Two Koniocellular Pathways in the Tree Shrew DLGN.

In the mammalian visual system, three functionally distinct parallel processing streams extend from the retina to the visual thalamus and then to the visual cortex: magnocellular (M), parvocellular (P), and koniocellular (K). Tree shrews (Tupaia belangeri), a preprimate species, provide an advantageous model to study the K pathway in isolation because, while M and P pathways remain mixed in Lamina 1 (L1), L2, L4, and L5 of the lateral geniculate nucleus (LGN), L3 and L6 receive strictly K-input from the contralateral eye. Additionally, K-input laminae selectively receive glutamatergic axons from the superior colliculus. To reveal how cellular and synaptic properties of K geniculate laminae may differ from M/P laminae and how tectal input may shape the K relay to the cortex, we studied the morphology and connectivity of retinal and tectal terminals in pathway-specific laminae. While confirming that K laminae relay cells contain calbindin, we also found its expression in GABAergic cells across all laminae. No cell-type or lamina specificity was observed for parvalbumin. Ultrastructurally, retinal terminals are morphologically distinct in M/P versus K laminae. Tectogeniculate axons in L3 and L6 resemble retinal terminals in their morphology and synaptic targets, while corticogeniculate terminals are sparse in L6. VGluT2, the molecular marker for large-sized driver terminals, is expressed prominently in one of the three tectal cell types that project to LGN. Morphological differences in synaptic circuitry between L3 and L6 provide further evidence that two geniculate K laminae are differentially innervated to relay distinct sets of information to the cortex.