Dendritic Spines in Early Postnatal Fragile X Mice Are Insensitive to Novel Sensory Experience.

Autism spectrum disorders are often associated with atypical sensory processing and sensory hypersensitivity, which can lead to maladaptive behaviors, such as tactile defensiveness. Such altered sensory perception in autism spectrum disorders could arise from disruptions in experience-dependent maturation of circuits during early brain development. Here, we tested the hypothesis that synaptic structures of primary somatosensory cortex (S1) neurons in Fragile X syndrome (FXS), which is a common inherited cause of autism, are not modulated by novel sensory information during development. We used chronic in vivo two-photon microscopy to image dendritic spines and axon "en passant" boutons of layer 2/3 pyramidal neurons in S1 of male and female WT and Fmr1 KO mice, a model of FXS. We found that a brief (overnight) exposure to dramatically enhance sensory inputs in the second postnatal week led to a significant increase in spine density in WT mice, but not in Fmr1 KO mice. In contrast, axon "en passant" boutons dynamics were impervious to this novel sensory experience in mice of both genotypes. We surmise that the inability of Fmr1 KO mice to modulate postsynaptic dynamics in response to increased sensory input, at a time when sensory information processing first comes online in S1 cortex, could play a role in altered sensory processing in FXS.SIGNIFICANCE STATEMENT Very few longitudinal in vivo imaging studies have investigated synaptic structure and dynamics in early postnatal mice. Moreover, those studies tend to focus on the effects of sensory input deprivation, a process that rarely occurs during normal brain development. Early postnatal imaging experiments are critical because a variety of neurodevelopmental disorders, including those characterized by autism, could result from alterations in how circuits are shaped by incoming sensory inputs during critical periods of development. In this study, we focused on a mouse model of Fragile X syndrome and demonstrate how dendritic spines are insensitive to a brief period of novel sensory experience.