PROJECT SUMMARY The opioid epidemic in the United States has reached unprecedented levels, and new treatments for opioid and heroin addiction are desperately needed. Heroin abuse is traditionally treated with opioid replacement therapies like methadone. However, these medications have negative side effects such as abuse liability and respiratory depression, suggesting the need for novel, safe pharmacotherapeutic medications to treat opioid use disorder. There is recent evidence that heroin produces its rewarding and addictive effects at least partially through activation of the mesolimbic dopamine system, and that heroin seeking and intake in preclinical addiction models can be modulated by treatments that target dopamine receptors. Specifically, dopamine D3 receptors have received considerable attention, as studies have shown D3 receptor antagonists reduce cue- induced reinstatement, a model of relapse-like behavior, for nicotine, alcohol, cocaine, and opioids. These findings suggest that D3 receptors may be a potential target for pharmacotherapies to treat heroin abuse and that D3 receptor antagonists may decrease relapse vulnerability. Therefore, this study aims to investigate (1) if acute and/or chronic administration of D3 receptor specific antagonists can decrease cue-induced heroin reinstatement in male and female rats and (2) if D3 receptor alterations after chronic exposure to heroin and/or extinction are driving relapse vulnerability. Because previous studies have shown that D3 receptor antagonists have therapeutic efficacy after acute administration, in Specific Aim 1 we will determine if chronic administration of a D3 antagonist is more effective than acute administration and if male and female rats demonstrate decreased reinstatement responding after D3 receptor antagonist administration. Our preliminary data suggest that D3 autoreceptors in the nucleus accumbens are overactive after chronic heroin exposure. Therefore, in Specific Aim 2, we will determine if this increase in D3 receptor activity is maintained throughout extinction and if females show the same alterations. Collectively, the proposed studies will provide insight into the potential for D3 receptor antagonists to decrease relapse vulnerability as well as examine a potential receptor target for future pharmacotherapeutic development. Further, the proposed studies will provide training in behavioral and neurochemical assays as well as data analysis, interpretation, and dissemination through manuscripts and presentations, which will serve as career development opportunities for the applicant.