Morphological and neurochemical comparisons between pulvinar and V1 projections to V2

Abstract

The flow of visual information is clear at the earliest stages: the retina provides the driving (main signature) activity for the lateral geniculate nucleus (LGN), which, in turn, drives the primary visual cortex (V1). These driving pathways can be distinguished anatomically from other modulatory pathways that innervate LGN and V1. The path of visual information after V1, however, is less clear. There are two primary feedforward projections to the secondary visual cortex (V2), one from the lateral/inferior pulvinar and the other from V1. Because both lateral/inferior pulvinar and V2 can't be driven visually following V1 removal, either or both of these inputs to V2 could be drivers. Retinogeniculate and geniculocortical projections are privileged over modulatory projections by their layer of termination, bouton size, and the presence of vesicular glutamate transporter 2 (Vglut2) or parvalbumin (PV). It has been suggested that such properties might also distinguish drivers from modulators in extrastriate cortex. We tested this hypothesis by comparing lateral pulvinar-to-V2 and V1-to-V2 projections with LGN-to-V1 projections. We found that V1 and lateral pulvinar projections to V2 are similar in that they target the same layers and lack PV. Projections from pulvinar to V2, however, bear a greater similarity to projections from LGN to V1 because of their larger bouton sizes (measured at the same location in V2) and positive staining for Vglut2. These data lend support to the hypothesis that the pulvinar could act as a driver for V2.

Reduced density of geniculocortical terminals in foveal layer 4A in the macaque primary visual cortex: relationship to S-cone density

“…In Western blot analysis, it appears as an individual band of ~5 kDa. The staining pattern was identical to the data published for macaques in previous studies that used the same monoclonal VGlut2 antibody (Marion et al 2013; Garcia-Marin et al 2013; Balaram et al 2013). …”

Distribution of vesicular glutamate transporter 2 (VGluT2) in the primary visual cortex of the macaque and human

“…1) and this corresponds to the known difference in size between the thalamo-cortical and intracortical terminals (Nahnami and Erisir, 2005). It has also been shown recently that the pulvinar terminals in V2 are smaller than those from the LGN to layer 4C of V1 (Marion et al, 2011). In the region of the V1/V2 border, we found that the VGluT2-ir puncta were of different sizes.…”

Differential expression of vesicular glutamate transporters 1 and 2 may identify distinct modes of glutamatergic transmission in the macaque visual system

“…Layers 1, 2 and 3 stained strongly for VGLUT1 and weakly for VGLUT2, appearing almost continuous through the extent of V2. Layer 4 contained strong VGLUT2 reactivity, which resembled patterns of pulvinar terminations in V2 (Levitt et al, 1995; Marion et al, 2012, Ogren and Hendrickson, 1977) but relatively weak VGLUT1 staining throughout V2. Layer 5 showed slightly weaker VGLUT1 labeling than layer 4, and diffuse labeling of VGLUT2.…”

“…Layer 4 of MT, where V1 projections terminate, showed dense distributions of VGLUT2 positive terminals as well. Layer 4 of V2, which is the primary recipient of feedforward projections from the lateral and inferior pulvinar, as well as V1, also showed dense VGLUT2 terminal labeling as well (Marion et al, 2012). Feedback or modulatory projections from V1 primarily arise in layers 5 and 6 and terminate diffusely in the LGN and superior colliculus, as described above, and these projections are dominated by VGLUT1.…”