p75 neurotrophin receptor in pre-adolescent prefrontal PV interneurons promotes cognitive flexibility in adult mice

Parvalbumin (PV)-positive GABAergic cells provide robust perisomatic inhibition to neighboring pyramidal neurons and regulate brain oscillations. Alterations in PV interneuron connectivity and function in the medial prefrontal cortex (PFC) have been consistently reported in psychiatric disorders associated with cognitive rigidity, suggesting that PV cell deficits could be a core cellular phenotype in these disorders. p75 neurotrophin receptor (p75NTR) regulates the time course of PV cell maturation in a cell-autonomous fashion. Whether p75NTR expression during postnatal development affects adult prefrontal PV cell connectivity and cognitive function is unknown. We generated transgenic mice with conditional knockout (cKO) of p75NTR in postnatal PV cells. We analysed PV cell connectivity and recruitment following a tail pinch, by immunolabeling and confocal imaging, in naive mice or following p75NTR re-expression in pre- or post-adolescent mice using Cre-dependent viral vectors. Cognitive flexibility was evaluated using behavioral tests. PV cell-specific p75NTR deletion increased both PV cell synapse density and the number of PV cells surrounded by perineuronal nets, a marker of mature PV cells, in adult PFC but not visual cortex. Both phenotypes were rescued by viral-mediated re-introduction of p75NTR in pre-adolescent but not in post-adolescent PFC. Prefrontal cortical PV cells failed to upregulated c-Fos following a tail-pinch stimulation in adult cKO mice. Finally, cKO mice showed impaired fear memory extinction learning as well as deficits in a rule set-shifting task. These findings suggest that p75NTR expression in adolescent PV cells contributes to the finetuning of their connectivity and promotes cognitive flexibility in adulthood.