Ultrastructure of geniculocortical synaptic connections in the tree shrew striate cortex

Abstract

To determine whether thalamocortical synaptic circuits differ across cortical areas, we examined the ultrastructure of geniculocortical terminals in the tree shrew striate cortex in order to directly compare the characteristics of these terminals to that of pulvinocortical terminals (examined previously in the temporal cortex of the same species, Chomsung et al. Cerebral Cortex 2010). Tree shrews are considered to represent a prototype of early prosimian primates, but are unique in that sublaminae of striate cortex layer IV respond preferentially to light onset (IVa) or offset (IVb). We examined geniculocortical inputs to these two sublayers labeled by tracer or virus injections, or an antibody against the type 2 vesicular glutamate antibody (vGLUT2). We found that layer IV geniculocortical terminals, as well as their postsynaptic targets, were significantly larger than pulvinocortical terminals and their postsynaptic targets. In addition, we found that 9–10% of geniculocortical terminals in each sublamina contacted GABAergic interneurons, whereas pulvinocortical terminals were not found to contact any interneurons. Moreover, we found that the majority of geniculocortical terminals in both IVa and IVb contained dendritic protrusions, while pulvinocortical terminals do not contain these structures. Finally, we found that synaptopodin, a protein uniquely associated with the spine apparatus, and telencephalin (TLCN, or Intercellular Adhesion Molecule type 5, ICAM5), a protein associated with maturation of dendritic spines, are largely excluded from geniculocortical recipient layers of the striate cortex. Together, our results suggest major differences in the synaptic organization of thalamocortical pathways in striate and extrastriate areas.

Ultrastructural analysis of parvalbumin synapses in human dorsolateral prefrontal cortex

“…The PV‐IR Type I synapses identified in the current study are also unlikely to arise from the amygdala, as the DLPFC receives sparse input from the amygdala (Barbas & De Olmos, ; Miyashita, Ichinohe, & Rockland, ), and PV immunoreactivity within the amygdala appears to be largely restricted to local circuit neurons (Mascagni, Muly, Rainnie, & McDonald, ; Sorvari, Soininen, Paljärvi, Karkola, & Pitkänen, ). However, substantial evidence across multiple species indicates that cortical PV Type I synapses originate from the thalamus (Blümcke, Hof, Morrison, & Celio, ; DeFelipe & Jones, ; del Río & DeFelipe, ; Familtsev et al, ; Freund, Martin, Soltescz, Somogyi, & Whitteridge, ; Freund, Martin, & Whitteridge, ; Jones, ; Jones & Hendry, ; Molinari et al, ; Negyessy & Goldman‐Rakic, ). Thus, although the present study cannot definitively determine that these are exclusively thalamocortical synapses, substantial evidence supports this interpretation.…”

Area-specific synapse structure in branched axons reveals a subcellular level of complexity in thalamocortical networks

“…We show that many Po and VPM boutons have an elongated, thick protrusion of the postsynaptic spine head invaginated into them. Two previous 2D electron microscopic studies of lateral geniculate nucleus axon synapses in the primary visual cortex of ferrets 43 and tree shrews 44 reported similar spine profiles, suggesting that spine intrusions may be common in mammalian thalamocortical synapses. While their precise functional significance remains to be determined; our data already provide some intriguing clues.…”

The Second Visual System of The Tree Shrew

“…As described in Section 4, the projections to striate cortex from dLGN layers that receive tectal input (Layers 3 and 6) are distinct from those dLGN layers that do not receive tectal input. Anterograde and retrograde tracer studies have shown that dLGN layers 3 and 6 terminate in the supragranular layers of striate cortex, whereas projections from the remaining dLGN layers (1, 2, 4, and 5) terminate primarily within Layer IV of the striate cortex (Carey, Fitzpatrick, & Diamond, ,; Raczkowski & Fitzpatrick, ; Usrey et al, ; Familtsev et al, ). Also, as described in Section 4, these two layers can be distinguished from the other dLGN layers by their RGC inputs (DeBruyn, ), by the receptive field characteristics of their neurons (Holdefer and Norton, 1995), by cytochrome oxidase histochemistry (Wong‐Riley & Norton, ) and by calcium binding protein labeling (Diamond et al, ).…”