PROJECT SUMMARY Our long-term goal is to understand how the structure and function of dendrites in the frontal cortex are affected by stress and antidepressants. Considerable evidence indicates that the action of fast-acting antidepressants such as ketamine relies on synaptic plasticity in the frontal cortex. An essential ingredient for plasticity is calcium influx in dendritic spines. However, to date, empirical data detailing how antidepressants may modulate calcium levels in dendritic spines in vivo are lacking. In preliminary studies, we used subcellular- resolution two-photon microscopy to visualize localized calcium transients in dendritic spines in the medial frontal cortex of the mouse in vivo. We found that the administration of the fast-acting antidepressant ketamine leads to an acute elevation of calcium signals in apical dendritic spines, due to a suppression of dendritic inhibition. On the basis of the preliminary results and prior research, we hypothesize that the enhanced synaptic calcium signal is a necessary step towards ketamine’s antidepressant action. In this project, we will determine the interactive effects of stress and ketamine on synaptic calcium signals (Aim 1). We will track morphology and calcium signals in the same dendritic spines to ask if the calcium elevations precede structural remodeling (Aim 2). We will determine if spines receiving inputs from specific long-range afferent regions are selectively targeted (Aim 3). Together, the results are expected to provide insights into the biological basis of ketamine’s action. As calcium is an essential checkpoint for synaptic plasticity in the frontal cortex, delineating its role in antidepressant action will accelerate the evaluation of candidate compounds and the development of more targeted treatments.