PROJECT SUMMARY Neural circuits are comprised of a rich network of neurons and associated glia that communicate through chemical signals. These signals are essential for proper development of neural circuits, and removal of these signals often impairs synapse development, circuit wiring, and circuit function. During development, animal experience drives activity-dependent remodeling of neural circuit structure and function during brief windows called critical periods. Critical periods are thought to set the ground state for neural circuit wiring, ensuring proper circuit connectivity and synaptic stability, though this hypothesis remains to be tested. Recent work indicates that astrocyte-derived signals regulate critical period closure across diverse circuits and animal models. In Drosophila, the motor circuit exhibits a critical period of activity-dependent structural remodeling at the embryo/larval transition. Astrocytes first associate with motor dendrites during peak motor plasticity, and ablating astrocytes is sufficient to extend the critical period. Genetic screening revealed that astrocyte-driven critical period closure is dependent on the expression of several cell surface molecules, including Neuroligin 2 (Nlg2) and Dally/GPC5. Astrocyte-specific knockdown of these signaling molecules extended critical period plasticity, whereas astrocyte-specific overexpression drove precocious critical period closure. To understand the mechanisms that establish and maintain neural circuits to drive animal behavior, it is important to examine the long-term consequences of altered critical period timing on circuit structure and function. I hypothesize that altering the timing of critical periods will induce long-term changes in motor circuit structure and function to produce distinct motor behaviors. I will use the complementary binary expression systems, Gal4/UAS or LexA/LexAop, to manipulate the expression of astrocyte-derived nlg2 and dally to extend (Aim 1) or precociously close (Aim 2) the motor critical period while I simultaneously assess motor circuit structure and function. Specifically, I will evaluate long-term changes to circuit connectivity, synaptic stability, motor output, and locomotor behavior by assessing these features at successively later stages spanning circuit development to the mature circuit. The completion of these experiments will provide insights into how altered critical period timing affects circuit development at the synaptic and cellular levels, and will directly link these changes to motor behavior. Thus, my goals meet priority area #4 of the BRAIN Initiative, “Demonstrating causality: Link brain activity to behavior with precise interventional tools that change neural circuit dynamics.” The role of astrocytes in closing critical periods has only recently been uncovered. This discovery opens a new intervention point for investigating the development and long-term maintenance of neural circuits. As this role of astrocyte...