Edward Han - US grants

Although significant advances in the treatment of opiate addiction have been made, relapse to opiate use after abstinence continues to impede successful treatment, highlighting the need for efforts to dissect the mechanism of opiate-dependent changes in brain plasticity. Recent studies have attempted to determine the role of structural plasticity in drug-induced behavior with conflicting findings which may result from the complexity of the intracellular signaling mechanisms underlying structural plasticity of dendritic spines caused by drugs of abuse. In this application we propose to develop novel in vivo approaches that will allow us to image the dynamic structural and functional plasticity that is triggered following opiate exposure and that may play a role in the mechanisms underlying reinstatement of drug seeking. To determine the relationship between dendrite structure and the formation and retrieval of drug associated memories, we propose to investigate how morphine exposure in a novel context modifies hippocampal dendritic spine morphology. In vivo imaging is a powerful tool to track rewiring in the hippocampal neural network, at the level of individual spines, throughout the learning, acquisition, expression, extinction and subsequent reinstatement of morphine conditioned place preference (CPP). Ideally one would be able to observe neural networks, in real time, as morphine CPP and reinstatement take place. The advent of virtual reality training paradigms during two-photon imaging makes this combination of behavior and network surveillance possible. In vivo imaging will enable us to follow the dynamics of spine remodeling and allow us to determine whether spine changes are cause or consequence of CPP. In addition, we will be able to visualize whether alterations in dendritic spines persist even following extinction which would indicate that spine remodeling may help store the latent memory driving drug-context associations. Therefore, the goals proposed in this application are: 1) to use in vivo 2-photon imaging in the dorsal hippocampus to follow structural changes in dendritic spines in CA1 neurons during the acquisition and expression of morphine CPP, and following its reinstatement; 2) to implement novel virtual reality spatial navigation protocols that enable us to conduct structural and functional imaging analyses of hippocampal cells in vivo during the formation of morphine-context associations. Overall, in this proposal we will conduct in vivo imaging analyses in awake mice to elucidate the temporal dynamics of hippocampal dendritic spine remodeling and its relationship to the formation of drug-context associations that may play a role in the mechanisms underlying reinstatement of drug seeking. In addition we will implement novel virtual navigation approaches to examine hippocampal circuit dynamics during the formation of morphine-context associations. The imaging methods and behavioral training protocols pioneered in this grant will be widely disseminated to the addiction field to advance the boundaries of current research.

Project Summary Humans, like other animals, regularly modify behavior based on environmental context. This relies on the ability to discriminate between environments and develop strategies for maximizing rewards (or minimizing punishment) in a context-specific manner. A breakdown in this ability to change behavior depending on environment is prominent in dementia and Alzheimer's disease. Our central objective is to identify the specific neuronal circuits and activity dynamics required for acquiring goal-oriented behaviors in novel environments. We focus on the hippocampus, a region critical for discriminating between environments and necessary for encoding certain types of behavior. Our central hypothesis is that cell-type specific inhibitory circuits regulate the pyramidal network dynamics that encode goal-oriented behavior. Specifically, we use in vivo two-photon calcium imaging to visualize the activity of genetically-defined subsets of hippocampal CA1 neurons as mice complete goal-oriented tasks in virtual reality (VR) environments, using water rewards for motivation (Arriaga and Han, J. Neurosci., 2017). With this system, we recently found that both parvalbumin (PV)- and somatostatin (SOM)-expressing inhibitory interneurons are strongly suppressed in novel environments, with gradual recovery of activity over days as task performance increases (Arriaga and Han, eLife, 2019). In Aim 1, we will use a combination of imaging, behavior, and correlative functional and immunolabeling microscopy to define putative disinhibitory VIP+ neurons activated in novel environments. In Aim 2, we will define the kinetics of excitatory network reorganization in novel environments during goal-oriented behavior. If inhibitory activity plays a major role in controlling the encoding of information in excitatory networks, we should see similar kinetics in activity dynamics across the two networks, i.e. slow stabilization over days. We will track individual pyramidal neurons during task-engaged behavior in novel environments to define activity dynamics of the excitatory network. To facilitate this goal, we have developed a neural network-based decoder that tracks the contribution of individual neurons to population position coding across days. In Aim 3, we will determine the necessity of inhibition suppression and disinhibition activation for goal-oriented behavior and pyramidal network reconfiguration. We will test this by chemogenetically restoring inhibitory SOM+ and PV+ interneuron activity (separately), or silencing PV+ neurons, in novel environments and compare task performance with control mice. To illuminate possible circuit mechanisms downstream of inhibitory activity manipulation, we will image excitatory neuron activity to evaluate alterations in network reorganization as defined in Aim 2. This contribution is significant because it promises to link cell type-specific inhibitory activity with novelty-induced, pyramidal network reorganization and goal-oriented behavior in vivo. These studies may lead to new circuit- targeted approaches to enhance network function for the treatment of behavioral impairment associated with many cognitive disorders and neurodegenerative diseases.