PROJECT SUMMARY/ABSTRACT Pain is a significant public health problem associated not only with unpleasant subjective sensations, but also with behavioral depression and functional impairment that reduce normal activities of daily living (e.g. eating, walking, working) and overall quality of life. Treatment and management strategies have become scarce as current medications have not been effective long-term and may come with a list of side effects that limits their use and make it difficult for patient compliance. High-efficacy opioids such as morphine which bind the mu opioid receptor (MOR) have a historical place for the effective treatment of pain but their use has been limited due to side effects and the opioid crisis. While the application acknowledges this, this effort neglects an alternative path for development of safe and effective analgesics: the further development of low-efficacy MOR agonists with sufficient efficacy to produce clinically effective analgesia with fewer and/or less severe side effects than high- efficacy MOR agonists. The proposed project aims to utilize a novel and highly flexible strategy for selective and precise control of net efficacy to activate MOR by testing a series of fentanyl and naltrexone (Fent/NTX) mixtures and novel MOR selective compounds. Special focus is proposed on pain-depressed behaviors as these are clinically translatable. Aim 1 proposes to study climbing as the pain-depressed behavioral endpoint. Five Fent/NTX mixtures that range from high to low net efficacy at MOR and three novel compounds with MOR selectivity and graded efficacy will be studied in the presence and absence of intraperitoneal (IP) lactic acid used as the acute noxious stimulus. Aim 2 proposes to study dopamine changes in the nucleus accumbens using the in vivo imaging fluorimetry technique dLight. This will be done using a subset of Fent/NTX mixtures to asses acute pain induced by IP lactic acid and pain-independent neurochemical changes. Aim 3 proposes to further study climbing as a pain-depressed behavior in a series of chronic pain studies. Separate groups of mice will receive paw incision to model postsurgical pain, intraplantar complete Freund’s adjuvant (CFA) to model inflammatory pain, repeated injection of the chemotherapy paclitaxel (PAC), and spared nerve injury (SNI) to model neuropathic pain. As a whole, the proposed project aims to investigate the effects of Fent/NTX mixtures as tools to investigate efficacy as a determinant of opioid treatment for acute pain-related behavioral depression and will set the stage to study these mixtures in chronic pain states as a means for better preclinical to clinical translation.