PROJECT SUMMARY/ABSTRACT – PROJECT 1 Project 1 focuses on several transcription factors that mediate the lasting actions of stimulants and opioids in the nucleus accumbens (NAc) and dorsal striatum. Virtually all prior studies of transcription factors in drug addiction used a candidate approach. This includes ∆FOSB and CREB, which are among the best studied transcription factors in addiction and first investigated based on “hunches” of their involvement. This is in marked contrast to a wholly unbiased approach to deduce, in an open-ended manner, those factors that are most important in the addiction phenotype. During the first funding period, we utilized this approach by taking advantage of large-scale RNAseq datasets, from rodent models and humans with SUDs, to identify those factors predicted to be important for lasting drug actions. Importantly, while these datasets identified numerous transcription factors not previously implicated in addiction, they also validated a central role for ∆FOSB, CREB, and other factors studied previously. We confirmed predictions for new factors by defining a cell-type-specific role for RXRa, E2F3A, ZFP189, and others in mediating cocaine and opioid action in NAc. We now propose to complete characterization of RXRa, which plays an especially important role and also provides a translational opportunity based on small molecule antagonists that could be used clinically. Our data show robust actions of RXRa in NAc neurons in both male and female mice and rats in controlling neuronal physiology as well as transcriptional and behavioral responses to cocaine and opioids, importantly, including the demonstration that an RXRa inhibitor reduces motivation to SA cocaine. We will now further characterize RXRa mechanisms by use of CUT&RUN—enabling the first mapping of transcription factor binding genome-wide in NAc—as well as the binding of several other factors, notably, ∆FOSB and CREB. Our goal is to identify key target genes for each factor in a cell-type-specific manner. Finally, Project 1 is building a novel pipeline to link these transcript- tion factors not only to cell activity in brain slices but also to cell activity in awake, behaving animals and to downstream effects on the functioning of the broader brain reward circuitry. Together, this work is revealing fundamentally new transcriptional mechanisms by which cocaine and opioids produce an addicted state.