Abstract The overall goal of this research is to better understand how endogenous opioids control the analgesic properties of the central noradrenergic system. Endogenous opioid systems provide powerful inhibition of locus coeruleus noradrenergic neurons and acute activation of mu opioid receptors in the locus coeruleus is antinociceptive. Therefore, we hypothesized that this endogenous mu opioid receptor-mediated inhibition could be critical to how the locus coeruleus modulates pain. Using conditional knockout and rescue of locus coeruleus-mu opioid receptor signaling, we show that the presence of these receptors in locus coeruleus neurons following neuropathic injury can reverse the expression of mechanical allodynia and thermal hyperalgesia. However, it is it is unknown whether the locus coeruleus-mu opioid receptor system is differentially modulated in pain states. This research focuses on understanding the mechanisms by which endogenous opioids inhibit the locus coeruleus noradrenergic system to promote endogenous analgesia and how chronic neuropathic injury disrupts this system. The central hypothesis of this proposal is that loss of mu opioid receptor-mediated locus coeruleus inhibition following long-term neuropathic injury promotes and maintains chronic pain. The first aim of this proposal will use neurochemical and gene expression studies to determine how endogenous opioid ligand and receptor systems in the locus coeruleus evolve following long-term neuropathic injury. The second aim seeks to understand whether projections from the locus coeruleus to the medial prefrontal cortex are selectively disinhibited after injury. This aim seeks to determine whether opioid sensitivity in these neurons is decreased in chronic pain. To do so we will use a high-throughput calcium imaging assay and in vivo optogenetics test the function of locus coeruleus neurons that project to the medial prefrontal cortex. The final aim seeks to identify non-opioid strategies for inhibiting the locus coeruleus to discover new analgesic targets. These studies will define the role of locus coeruleus mu opioid receptors 1) in pain from neuropathic injury, 2) along projections to the medial prefrontal cortex, and 3) identify mechanisms to suppress pain-generating locus coeruleus activity. This information will be critical for translational research targeting the noradrenergic system in the treatment of pain and neuropsychiatric disorders.