Project Summary/Abstract DNA is generally thought of as stable. However, retrotransposons are mobile elements that result in new copy insertions in DNA. Emerging evidence suggests retrotransposons can be affected by changes in the environment, suggesting a novel mechanism by which exposure to early life insults can cause enduring effects. The long-term goal of this proposal is to provide enhanced training that will enable the applicant to establish novel mechanisms by which reward circuitry is affected. Aims in this proposal will test the extent to which the retrotransposon, long-interspersed element 1 (L1) is impacted by environmental conditions and the impact on behavior related to substance use disorder (SUD) in adulthood. In the K99 phase, Aim1 will utilize innovative molecular-genetic approaches to determine where de novo L1 insertions reside within the basolateral amygdala (BLA) due to early life adversity (ELA). Aim 2 will utilize magnetic-activated cell sorting (MACS) to determine whether de novo L1 insertions reside within a particular cell type. In the R00 independent phase, the applicant will leverage this training to advance a research program that expands to studying L1 throughout development and the impact on morphine self-administration. The innovative approach of this proposal comes from the intersection of technical and conceptual advances in molecular biology, behavioral neuroendocrinology, and addiction neuroscience. Expanding basic knowledge of mechanisms by which the reward pathway is impacted can provide a critical foundation for future design of treatments for substance use disorder and other neuropsychiatric disorders.