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Francisco, for fostering in her the joy for science and the scientific process, during the gap years after her undergraduate education. We would like to thank the following students in helping us get started with this project and for technical

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Cortical neuronal circuits along the sensorimotor pathways are shaped by experience during critical periods of heightened plasticity in early postnatal development. After closure of critical periods, measured histologically by the formation and maintenance of extracellular matrix structures called perineuronal nets (PNNs), the adult mouse brain exhibits restricted plasticity and maturity. Mature PNNs are typically considered to be stable structures that restrict synaptic plasticity on cortical parvalbumin+ GABAergic neurons. Changes in environment (i.e. novel behavioral training) or social contexts (i.e. motherhood) are known to elicit synaptic plasticity in relevant neural circuitry. However, little is known about concomitant changes in the PNNs surrounding the cortical parvalbumin+ GABAergic neurons. Here, we show novel changes in PNN density in the primary somatosensory cortex (SS1) of adult female mice after maternal experience, using systematic microscopy analysis of a whole brain region. On average, PNNs were increased in the right barrel field and decreased in the left forelimb regions. Individual mice had left hemisphere dominance in PNN density. Using adult female mice deficient in methyl-CpG-binding protein 2 (MECP2), an epigenetic regulator involved in regulating experience-dependent plasticity, we found that MECP2 is critical for this precise and dynamic expression of PNN. Adult naïve Mecp2-heterozygous females (Het) had increased PNN density in specific subregions in both hemispheres before maternal experience. The laterality in PNN expression seen in naïve Het was lost after maternal experience, suggesting possible intact mechanisms for plasticity. Together, our results identify subregion and hemisphere-specific alterations in PNN expression in adult females, suggesting extracellular matrix plasticity as a possible neurobiological mechanism for adult behaviors in rodents.

Challenging the brain with experiential richness creates tissue-level changes and synaptic plasticity, but the interjacent network level has not been accessible. We here show that environmental enrichment has unexpectedly far-reaching effects on network connectivity and multi-dimensional coding in the hippocampus. We present direct evidence that experience impacts local and global network connectivity, synchrony, and rhythmic dynamics. For this, we investigated the hippocampi from standard-housed mice (SD) and mice living in an enriched environment (ENR) using large-scale ex vivo recordings with a high-density microelectrode sensing array that with the unprecedented spatiotemporal resolution allowed simultaneous electrophysiological assessment across the entire circuit. In the absence of extrinsic electrical network stimulation, we found enhanced functional connectivity and high-dimensional coding in hippocampal-cortical networks of ENR mice. The mapped connectome illustrated a scale-free small-world topology and an ENR-induced resilience to random failures. ENR enhanced large-scale spatiotemporal firing patterns, which facilitated efficient pattern separation and boosted the information encoded in the firing phases of slow oscillatory rhythms. Given that essentially all electrophysiological studies on network behaviors have been done on animals housed in stimulus-poor conditions, our SD mice showed the expected normal functionality. The literature consequently underestimates the extent of spontaneous network activity and connectivity under truly physiological conditions. Our results pave the way to unveil fundamental mechanisms of experience-dependent enhancement in the hippocampal network underlying high brain functions and provide markers for large-scale network remodeling and metaplasticity.