Project Summary/Abstract Drug addiction is a leading cause of morbidity and mortality in the US. Opioid addiction in particular has become an epidemic with unprecedented overdose fatalities, half of which are caused by fentanyl. Our repertoire to treat opioid addiction is very limited, and the progress in finding effective treatments has stalled. Transcranial Magnetic Stimulation (TMS) is emerging as a potential therapeutic tool; it is FDA approved for depression, and a pilot study suggested a role of TMS in treating cocaine addiction. This proposal will investigate the parameters of TMS use in a mouse model of opioid addiction. Through magnetic pulses, TMS non-invasively activates cortical neurons in the targeted area, resulting in brain- wide changes. However, several questions remain unanswered including whether TMS induces long- lasting changes in downstream circuits beyond the targeted area, how different TMS parameters affect brain circuits, and whether TMS can reverse drug-induced changes and interrupt drug seeking. This proposal will address these questions using TMS of the olfactory bulb in a mouse model of fentanyl self-administration paired with olfactory cues. The hypothesis is that TMS of the olfactory bulb induces plasticity changes in downstream circuits involved in learning and reward such as the piriform cortex and olfactory tubercle (part of the ventral striatum), and therefore can be used to reverse drug- induced plasticity in those areas and inhibit relapse to drug seeking induced by olfactory cues. The first aim will determine the extent of activation by TMS of the olfactory bulb on the bulb itself, downstream targets, and contiguous areas using confocal imaging of c-fos immunohistochemistry. The second aim will define the effects of different TMS parameters on downstream plasticity using brain slice electrophysiology combined with optogenetics. The third aim will develop and validate a novel mouse model of fentanyl vapor self-administration and olfactory-cue-induced relapse that is compatible with the TMS model. The last and fourth aim will study the effects of olfactory bulb TMS on fentanyl-induced plasticity in the olfactory tubercle and olfactory-cue-induced relapse to fentanyl seeking. This proposal is also crafted to help the Principal Investigator achieve his goal in launching an independent academic research program focused on using neuromodulation in the treatment of drug addiction. The career development plan includes hands-on and didactic learning of the skills necessary for accomplishing the K99 Aims (1 and 3). It also includes extensive professional development training such as mentorship, grant writing, networking, presenting scientific data, lab management, and preparing for job interviews. The institutional environment at the intramural research program at NIDA, the project primary location, furnishes all the necessary resources.