DIFFERENTIAL THALAMOCORTICAL INNERVATION AND PERINEURONAL NET ASSEMBLY DISTINGUISH PV CIRCUITS IN PRIMARY AND HIGHER-ORDER VISUAL CORTICAL AREAS

In the mammalian brain, accumulation of perineuronal nets (PNNs) around parvalbumin (PV)-expressing GABAergic interneurons (INs), close critical periods of plasticity. However, the underlying mechanisms are unknown. Previous work in the mouse primary visual cortex (V1) indicates that PNNs selectively affect thalamocortical (TC) inputs onto PV INs. Here, we found that, in adult mice, PNNs accumulate preferentially around PV INs on TC recipient layers of V1. Strikingly, in adjacent higher-order areas (such as V2M), PV INs are devoid of PNNs. We performed single and multiple patch-clamp recordings coupled with single-fiber optogenetic interrogation of TC synaptic transmission onto PV INs and pyramidal neurons (PNs). We found that endogenous accumulation of PNNs does not influence the maturation of PV IN excitability, nor the properties of synaptic transmission between PV INs and PNs. However, despite comparable single-fiber biophysical properties of TC transmission in the two areas, bulk TC recruitment of PV cells is stronger in V1 than V2M. This results in stronger feed-forward inhibition of PNs in V1 than V2M. This asymmetrical TC recruitment scheme was corroborated by anatomical analysis of TC fibers in the two areas. We are currently quantifying the precise TC innervation onto PNN-enriched and deprived PV INs using split-GFP fragments at TC-PV synapses. We are also testing the hypothesis that selective PNN accumulation in V1 prevents the saturation of massive TC recruitment of PV INs, thereby providing a synaptic and cellular mechanism that promotes stability at the cost of plasticity in primary sensory cortical areas.