PROJECT SUMMARY: Methamphetamine is a highly addictive drug that has major effects on the brain’s learning and memory systems. Methamphetamine use is commonly predicted by high use of legal psychostimulants such nicotine or alcohol, but the mechanisms underlying sequential use of alcohol, nicotine and methamphetamine are not well understood. Both nicotine and alcohol alter the function of brain circuits required for adaptive decisions, but the cellular and molecular mechanisms through which this occurs largely remain a mystery. Identifying the gene regulatory networks within select circuits in high alcohol or high nicotine seeking animals and deciphering how methamphetamine alters these networks is necessary to decode the fundamental molecular underpinnings through which methamphetamine influences maladaptive choice. However, the diversity of circuits and cells involved in these responses has confounded our ability to identify key molecular pathways. Our goal is to use a genes-circuits-behavior approach in Drosophila melanogaster to provide an innovative and holistic understanding of the core fundamental principles through which sequential use of psychostimulants alter the molecular landscape and neural dynamics of memory circuits to alter behavioral decisions. Transcriptional regulation within memory circuits is a fundamental process through which psychostimulants drive maladaptive changes in the brain function. This work provides the causal gene regulatory mechanisms through which high preference for alcohol or nicotine affects methamphetamine response in memory circuits. This has major implications for how co-use of psychostimulants alters the molecular landscape that drives drug-seeking behavior.