SUMMARYCortical sensory maps are remodeled during early life to adapt to the surrounding environment. Both sensory and contextual signals play an important role, but how these two inputs converge to sculpt developing thalamocortical circuits remains largely unknown. Here, we reveal layer 1 (L1) of primary auditory cortex (A1) to be a key hub where neuromodulatory and topographically-organized thalamic inputs meet to tune the cortical layers below. Inhibitory interneurons in L1 send narrowly descending projections to differentially modulate thalamic drive onto pyramidal and parvalbumin-expressing (PV) cells in L4, creating brief windows of intracolumnar activation. Silencing L1 (but not VIP) cells abolishes map plasticity during the tonotopic critical period. Instead, developmental transitions of nicotinic acetylcholine receptor (nAChR) sensitivity in these cells by the Lynx1 protein can be overridden to extend critical period closure. Notably, thalamocortical maps in L1 are themselves stable, serving as a scaffold for cortical plasticity throughout life.

Parvalbumin-positive (PV+) interneurons play a pivotal role in orchestrating windows of experience-dependent brain plasticity during development. Critical period closure is marked by the condensation of a perineuronal net (PNN) tightly enwrapping subsets of PV+ neurons, both acting as a molecular brake on plasticity and maintaining mature PV+ cell signaling. As much of the molecular organization of PNNs exists at length scales near or below the diffraction limit of light microscopy, we developed a superresolution imaging and analysis platform to visualize the structural organization of PNNs and the synaptic inputs perforating them in primary visual cortex. We identified a structural trajectory of PNN maturation featuring a range of net structures, which was accompanied by an increase in Synaptotagmin-2 (Syt2) signals on PV+ cells suggestive of increased inhibitory input between PV+ neurons. The same structural trajectory was followed by PNNs both during normal development and under conditions of critical period delay by total sensory deprivation or critical period acceleration by deletion of MeCP2, the causative gene for Rett syndrome, despite shifted maturation levels under these perturbations. Notably, superresolution imaging further revealed a decrease in Syt2 signals alongside an increase in vesicular glutamate transporter-2 signals on PV+ cells in MeCP2-deficient animals, suggesting weaker recurrent inhibitory input between PV+ neurons and stronger thalamocortical excitatory inputs onto PV+ cells. These results imply a latent imbalanced circuit signature that might promote cortical silencing in Rett syndrome before the functional regression of vision. parvalbumin interneuron | critical period | visual cortex | dark rearing | MeCP2

Summary This note describes nTracer, an ImageJ plug-in for user-guided, semi-automated tracing of multispectral fluorescent tissue samples. This approach allows for rapid and accurate reconstruction of whole cell morphology of large neuronal populations in densely labeled brains. Availability and implementation nTracer was written as a plug-in for the open source image processing software ImageJ. The software, instructional documentation, tutorial videos, sample image and sample tracing results are available at https://www.cai-lab.org/ntracer-tutorial. Supplementary information Supplementary data are available at Bioinformatics online.

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