PROJECT SUMMARY Repetitive transcranial magnetic stimulation (rTMS) is noninvasive method for brain stimulation and is an FDA- approved treatment for major depression and obsessive compulsive disorder. It also shows promise in treating numerous other neurological and psychiatric disorders. High frequency (HF) rTMS targeting prefrontal cortex (PFC), the original and most widely used paradigm, is thought to exert its therapeutic effects by enhancing cortical excitability. However, clinical outcomes following HF-rTMS treatment are variable, and the detailed mechanisms of action are not known. Previous mechanistic studies have been limited by a lack of established animal models of rTMS with strong face validity. Our lab has acquired the first rodent TMS coil capable of generating focal, suprathreshold stimulation of individual cortical regions in the rodent brain. I will use this coil to determine how in vivo chronic HF-rTMS modifies prefrontal excitatory neurons and which projection classes underlie improved behavioral outcomes. By combining rTMS with cutting edge neuroscience tools, I will test the hypothesis that HF-rTMS specifically induces structural plasticity in intratelencephalic (IT) circuits and that activation of these circuits underlies HF-rTMS-induced changes in behavior. In Aim 1, I will determine how and where chronic HF-rTMS induces synaptic changes by using sparse fluorescent labeling of excitatory neurons in PFC to quantify dendritic spine density. In Aim 2, I will use chemogenetic approaches to determine whether subclasses of prefrontal neurons underlie behavioral effects of HF-rTMS. This proposal addresses a pressing need to understand which circuits mediate the effects of HF-rTMS on behavior. This research will inform the rational design of more effective rTMS treatments that precisely target specific deficits underlying the pathophysiology of psychiatric disorders.