Imaging Plasticity

Many CPGs are capable of modifying neuronal structure, suggesting that neuronal structure can be modified by activity even in the adult brain. To investigate the molecular mechanisms underlying structural plasticity in the mammalian brain we have a long-standing collaboration with Peter So’s lab in the Department of Mechanical Engineering at MIT to develop multi-photon microscopy for large volume, high resolution imaging of dendritic arbor and synaptic structural dynamics in vivo. Using this system we have imaged and reconstructed the dendritic trees and axon collaterals of neurons in visual cortex of thy1-EGFP transgenic mice. These transgenic mice express EGFP in a random subset of neurons sparsely distributed within the superficial cortical layers that are optically accessible through surgically implanted cranial windows. The images show an abundance of detail, with the basal and apical dendrites instantly recognizable and spines clearly visible. This enables dendritic branch dynamics in individual neurons to be examined over several months. We were the first to reveal that dendritic arbor remodeling in the adult is restricted to inhibitory interneurons (Lee et al., PLoS Biology 2006), that interneuron remodeling is not a feature determined by cell lineage, but rather is imposed by the laminar cortical circuitry (Lee et al., PNAS 2008), and is elicited by sensory experience in an input-specific manner so as to facilitate or attenuate experience-dependent plasticity (Chen et al, Nature Neuroscience 2011). Further we showed that inhibitory dendrite remodeling is a common element of structural plasticity mechanisms built into the neocortical module (Chen et al., J. Neurosci. 2011).

Once interneurons were recognized as key players in activity-dependent circuit plasticity, a major obstacle to addressing questions regarding inhibitory synapse plasticity as well as how it may be coordinated with excitatory inputs at the dendritic level, has been the inability to visualize inhibitory synapses in vivo. We therefore expanded our high-resolution large volume imaging capabilities with incorporation of two-color multiphoton microscopy to simultaneously monitor both inhibitory synapse and dendritic spine remodeling across the entire dendritic arbor of cortical L2/3 pyramidal neurons in vivo during normal and altered sensory experience. This has allowed us to address, for the first time, fundamental questions about the interplay between excitatory and inhibitory synaptic transmission during normal adult brain function as well as experience-dependent plasticity (Chen et al., Neuron 2012). Currently we are working to integrate additional colors into our imaging set up to enable simultaneous tracking of multiple synaptic and cellular components. We are also interested in integrating Ca+2 imaging with our structural markers.