In Vivo Synaptic Imaging in Neocortex

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ABSTRACT Synapse addition and loss have been linked to learning throughout the brain. This has been well-documented in the neocortex using anatomical analysis of axonal boutons or dendritic spines in both fixed tissue and in vivo, using longitudinal imaging. However, it has been difficult to extrapolate the consequences of these synaptic changes without understanding how they lie within a defined network of cell-type specific contacts, requiring identification of the pre- and postsynaptic partners of the synapse. Here we will use in vivo imaging and multicolor fluorescence labeling of molecularly-defined pre- and postsynaptic neurons to monitor input- specific synapse addition and loss during sensory learning in a whisker-dependent task in mice. Synaptic contacts will be validated using post-hoc expansion microscopy and immunohistochemistry for nanoscale resolution. Analysis will be focused on learning-dependent reorganization of thalamic inputs from a higher-order thalamic nucleus, the posterior medial nucleus (POm) onto the dendrites of layer 5 (L5) pyramidal neurons that have been specifically implicated in experience-dependent plasticity. Quantitative, multicolor, in vivo imaging across different stages of learning will provide insight into how cortical circuits encode and are changed by salient sensory information.