PROJECT SUMMARY Adverse childhood experiences (ACEs), chronic stress, and isolation increase the odds of misusing opioids. In treatment-seeking people with opioid use disorder, exposure to more types of childhood trauma was inversely correlated with age at opioid initiation. Thus, people with ACEs begin misusing opioids earlier than people without ACEs. Current medications for opioid use disorder (MOUDs) significantly reduce overdose and instances of relapse. However, MOUDs are not sufficient. The number of synthetic opioid overdose deaths continues to rise and instances of OUD are climbing, illustrating the serious public health burden. Alternative medications that have unique non-opioid mechanisms of action could increase recovery and mitigate the abuse potential of prescription opioids. The serotonin (5-HT) system is richly expressed in many areas that modulate opioid reward and is dynamically altered by stress and adverse experiences. The 5-HT system and the receptors that transduce the signal are densely expressed in the mesocorticolimbic regions in human and rodent brains and modulate reinforcement learning. Specifically, the 5-HT2A and 5-HT2C receptors (5-HT2AR and 5-HT2CR) seem to have opposing effects within the mesocorticolimbic pathway and areas of the mPFC. However, new preliminary data from our research group identifies that a preferential 5-HT2AR partial agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI) reduces the reinforcing value of oxycodone in a novel, translationally relevant model of fentanyl self- administration. The ongoing opioid crisis calls for novel treatment strategies and innovative approaches to address the opioid epidemic. Furthermore, there is a critical need to determine how ACEs and chronic stress alter the rewarding aspects of synthetic opioids like fentanyl. Elucidating the neurobiological mechanisms that are altered during development and have downstream effects on opioid reinforcement helps to predict or identify at-risk groups who are more vulnerable to OUD. This application seeks support to investigate these mechanisms to ultimately reveal a novel and innovative neuropharmacological mechanism and identify a non-opioid receptor target to control opioid reinforcement. This project has the potential to strengthen the research environment at the University of Nebraska at Omaha by recruiting and exposing undergraduate students to translationally relevant models of OUD. In addition, the undergraduate students will be incorporated into all aspects of the proposed research projects to deliver a truly experiential training opportunity that fosters independent growth and professional development. Students engaged in this project will communicate their research projects through conference presentations and publications. At the end of this project, our primarily undergraduate research team will have determined the therapeutic utility of the 5-HT2AR in fentanyl reinforcement, characterized the expression of 5-HT2AR express...