p75 Neurotrophin Receptor Regulates the Timing of the Maturation of Cortical Parvalbumin Cell Connectivity and Promotes Ocular Dominance Plasticity in Adult Visual Cortex.

By virtue of their extensive axonal arborisation and perisomatic synaptic targeting, cortical inhibitory Parvalbumin (PV) cells strongly regulate principal cell output and plasticity. An interesting aspect of PV cell connectivity is its prolonged maturation time course, which is completed only by end of adolescence. The p75 neurotrophin receptor (p75NTR) regulates a wide range of cellular function, including apoptosis, neuronal process remodeling and synaptic plasticity, however its role on cortical circuit development is still not well understood, mainly because localizing p75NTR expression with cellular and temporal resolution has, so far, been challenging. Using RNAscope and a modified version of the Proximity Ligation Assay (PLA), we show that p75NTR mRNA and protein are expressed in cortical PV cells in the postnatal and adult brain. Further, p75NTR expression in PV cells decreases between postnatal day (P)14 and P26, at a time when PV cell synapse numbers increase dramatically. Conditional knockout of p75NTR in single PV neurons in cortical organotypic cultures and in PV cell networks in vivo leads to precocious formation of PV cell perisomatic innervation and perineural nets around PV cell somata, suggesting that p75NTR expression controls the timing of the maturation of PV cell connectivity in the adolescent cortex. Remarkably, we found that p75NTR is still expressed, albeit at low level, in PV cells in adult cortex. Interestingly, activation of p75NTR onto PV cells in adult visual cortex in vivo is sufficient to destabilize their connectivity and to reintroduce juvenile-like cortical plasticity following monocular deprivation. Altogether, our results show that p75NTR activation dynamically regulates PV cell connectivity, and represents a novel tool to foster brain plasticity in adults.