Characterization of the Morphological and Synaptic Properties of Terminals in Koniocellular versus Magnocellular/Parvocellular Parallel Pathways in the Lateral Geniculate Nucleus of the Tree Shrew (Tupaia belangeri)

In addition to its main excitatory input coming from the retina, the lateral geniculate nucleus (LGN) of the thalamus receives axons from the superior colliculus (SC). In tree shrews (Tupaia belangeri), these tectogeniculate (TG) projections terminate in the koniocellular laminae 3 and 6, but their precise role in this pathway is unknown. To address the circuitry that underlines the TG input regulation of geniculate relay, I studied the synaptic circuitry in koniocellular laminae and its differences from those in magnocellular and parvocellular laminae. In particular, I characterized the ultrastructural and connective properties of retinal and SC terminals across these laminae in the LGN of tree shrews. Electron microscopy analysis to categorize the morphological and synaptic characteristics of these LGN terminals revealed that the terminals in both koniocellular and magno/parvo laminae display a multi-modal distribution, indicating inputs from various origins. Immuno-EM experiments revealed that the largest size subpopulation of terminals contained VGLUT2 and formed large synaptic zones with thick postsynaptic density primarily onto dendrites in laminae 1 and 2, and primarily onto vesicle-filled, presumed interneuronal profiles in lamina 6. Triadic arrangements were seen in both sets of laminae, however, they were more prevalent in VGLUT2+ terminals in lamina 6. VGLUT2+ terminals in lamina 6 were smaller, had less visible protrusions, made less synaptic contacts, and consisted of two distinct subpopulations of terminal sizes. These findings provide evidence that the morphological and connective characteristics of synaptic circuitry in the tree shrew LGN laminae differ based on their parallel pathway segregation. Furthermore, the differences between these parallel pathways may be due, in part, by the TG inputs that project to koniocellular laminae.