SUMMARY OF PROPOSAL Modern antiretroviral therapies have transformed Human Immunodeficiency Virus (HIV) infection into a chronically manageable disease. However, a significant amount of people with HIV on antiretroviral therapies present with a spectrum of neurological symptoms. These HIV-associated neurocognitive disorders (HAND) remain an important aspect of modern HIV pathology, prognosis, and clinical management, raising the need for the discovery of adjuvant therapies. Clinical evidence also suggests that HAND can be aggravated by substance abuse, including opioids, which is highly prevalent in people with HIV. Understanding how opioid use contributes to cognitive impairment in HAND will help identify new approaches and drug targets to treat HAND in opioid users. For the past two decades, we have investigated how HIV neurotoxins and opioids contribute to HAND by regulating the chemokine receptor CXCR4 in the central nervous system. We have demonstrated that proper CXCR4 signaling via its natural ligand (the chemokine CXCL12) is not only neuroprotective, but it can also rescue structural (i.e. dendritic spines loss) and functional (i.e. cognitive flexibility) deficits in an animal model of HAND. Further, we discovered that this pathway is hijacked by viral proteins and inhibited by µ-opioid receptor agonists, which revealed a new mechanism whereby opioid use can aggravate HAND. Overall, our body of work demonstrates that the CXCL12/CXCR4 axis is tightly integrated into homeostatic and plasticity processes in cortical neurons, and disruption of CXCR4 signaling via HIV proteins, inflammatory mediators, and opioids is an important component of HAND. For the next (and last) period of this award, we plan to capitalize on this knowledge and other recent findings and fill key gaps in the field that will help us assess the therapeutic value of these discoveries. Thanks to new collaborations, we shifted our focus from molecular mechanisms to network level function – with a main interest in the role of cortical interneurons in HAND. Notably, cortical interneurons dysfunction has emerged as a key factor in neuropsychiatric conditions linked to substance use disorders. Our objectives for the next five years are to determine how interneurons contribute to CXCL12’s regulation of neuronal network activity and cognition in models of HAND, and if the chemokine’s effects are inhibited by morphine via local circuit modulation. Studies in Aim 1 will examine synaptic plasticity deficits that are indicative of microcircuit dysfunction in the medial PFC of WT and HIV-tg rats, and if these network-level deficits are reversed by CXCL12 treatment. Aim 2 studies will examine how dendritic-targeting cortical interneurons that express CXCR4 (namely, SST+ inhibitory neurons) modulate the activity of local networks to enhance neuronal connectivity, and how these processes are affected by inhibitory neurons that express µ-opioid receptors. In...