Spine dynamics of PSD-95-deficient neurons in the visual cortex link silent synapses to structural cortical plasticity.

Critical periods (CPs) are time windows of heightened brain plasticity during which experience refines synaptic connections to achieve mature functionality. At glutamatergic synapses on dendritic spines of principal cortical neurons, the maturation is largely governed by postsynaptic density protein-95 (PSD-95)-dependent synaptic incorporation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors into nascent AMPA-receptor silent synapses. Consequently, in mouse primary visual cortex (V1), impaired silent synapse maturation in PSD-95-deficient neurons prevents the closure of the CP for juvenile ocular dominance plasticity (jODP). A structural hallmark of jODP is increased spine elimination, induced by brief monocular deprivation (MD). However, it is unknown whether impaired silent synapse maturation facilitates spine elimination and also preserves juvenile structural plasticity. Using two-photon microscopy, we assessed spine dynamics in apical dendrites of layer 2/3 pyramidal neurons (PNs) in binocular V1 during ODP in awake adult mice. Under basal conditions, spine formation and elimination ratios were similar between PSD-95 knockout (KO) and wild-type (WT) mice. However, a brief MD affected spine dynamics only in KO mice, where MD doubled spine elimination, primarily affecting newly formed spines, and caused a net reduction in spine density similar to what has been observed during jODP in WT mice. A similar increase in spine elimination after MD occurred if PSD-95 was knocked down in single PNs of layer 2/3. Thus, structural plasticity is dictated cell autonomously by PSD-95 in vivo in awake mice. Loss of PSD-95 preserves hallmark features of spine dynamics in jODP into adulthood, revealing a functional link of PSD-95 for experience-dependent synapse maturation and stabilization during CPs.

Transgenerational Transmission of Enhanced Ocular Dominance Plasticity from Enriched Mice to Their Non-enriched Offspring.

In recent years, evidence has accumulated that non-Mendelian transgenerational inheritance of qualities acquired through experience is possible. In particular, it has been shown that raising rodents in a so-called enriched environment (EE) can not only modify the animals' behavior and increase their susceptibility to activity-dependent neuronal network changes, but also influences both behavior and neuronal plasticity of the non-enriched offspring. Here, we tested whether such a transgenerational transmission can also be observed in the primary visual cortex (V1) using ocular dominance (OD) plasticity after monocular deprivation (MD) as a paradigm. Whereas OD plasticity after 7 d of MD is absent in standard-cage (SC) raised mice beyond postnatal day (P)110, it is present lifelong in EE-raised mice. Using intrinsic signal optical imaging to visualize cortical activity, we confirm these previous observations and additionally show that OD plasticity is not only preserved in adult EE mice but also in their adult non-enriched offspring: mice born to enriched parents, but raised in SCs at least until P110 displayed similar OD shifts toward the open eye after 7 d of MD as age-matched EE-raised animals. Furthermore, testing the offspring of EE-female versus EE-males with SC-mating partners revealed that only pups of EE-females, but not of EE-males, preserved OD plasticity into adulthood, suggesting that the life experiences of the mother have a greater impact on the continued V1 plasticity of the offspring. The OD plasticity of the non-enriched pups of EE-mothers was, however, mechanistically different from that of non-enriched pups of EE-parents or EE mice.

Long-Term Visual Training Increases Visual Acuity and Long-Term Monocular Deprivation Promotes Ocular Dominance Plasticity in Adult Standard Cage-Raised Mice.

For routine behavioral tasks, mice predominantly rely on olfactory cues and tactile information. In contrast, their visual capabilities appear rather restricted, raising the question whether they can improve if vision gets more behaviorally relevant. We therefore performed long-term training using the visual water task (VWT): adult standard cage (SC)-raised mice were trained to swim toward a rewarded grating stimulus so that using visual information avoided excessive swimming toward nonrewarded stimuli. Indeed, and in contrast to old mice raised in a generally enriched environment (Greifzu et al., 2016), long-term VWT training increased visual acuity (VA) on average by more than 30% to 0.82 cycles per degree (cyc/deg). In an individual animal, VA even increased to 1.49 cyc/deg, i.e., beyond the rat range of VAs. Since visual experience enhances the spatial frequency threshold of the optomotor (OPT) reflex of the open eye after monocular deprivation (MD), we also quantified monocular vision after VWT training. Monocular VA did not increase reliably, and eye reopening did not initiate a decline to pre-MD values as observed by optomotry; VA values rather increased by continued VWT training. Thus, optomotry and VWT measure different parameters of mouse spatial vision. Finally, we tested whether long-term MD induced ocular dominance (OD) plasticity in the visual cortex of adult [postnatal day (P)162-P182] SC-raised mice. This was indeed the case: 40-50 days of MD induced OD shifts toward the open eye in both VWT-trained and, surprisingly, also in age-matched mice without VWT training. These data indicate that (1) long-term VWT training increases adult mouse VA, and (2) long-term MD induces OD shifts also in adult SC-raised mice.