PROJECT SUMMARY. Recent epidemiological reports indicate that cocaine use increased by 45% from 2013 to 2018. Cocaine-induced increase in striatal dopamine levels is linked to its rewarding effects. However, the striatal neural circuits driving cocaine abuse are not clearly defined. Thus, there is a critical need to delin- eate how the striatal circuits downstream of cocaine-induced dopamine release control cocaine seeking and taking. The overall objective of this proposal is to determine the mechanisms by which cocaine affects GABA transmission between the two principal neurons of the striatum, the D1-MSNs and D2-MSNs, to ultimately regulate drug seeking and taking. Our central hypothesis is that cocaine-enhances levels of the opioid peptide enkephalin, which acts via mu-opioid receptors (MOR) expressed in axon collaterals of D1- and D2-MSNs to facilitate cocaine seeking and taking. We will test this in two specific aims: Aim 1: Determine the mechanisms for how cocaine affects plasticity of intra-striatal GABA trans- mission. Based on our preliminary data, we hypothesize that a history of cocaine increases enkephalin re- lease from D2-MSNs, which induces MOR-dependent long-term depression of intra-striatal GABA transmis- sion onto D1-MSNs. We will test this by performing whole cell electrophysiology in MSN-selective MOR and enkephalin knockouts with a history of cocaine or saline exposure, and record GABA transmission between MSNs and MSN excitability. Pharmacology will be used to infer heightened enkephalin tone in electrophysi- ology recordings, and this will be confirmed by RNAscope, immunohistochemistry, Western blot and our MSN- selective enkephalin knockout. Aim 2: Determine how opioid regulation of intra-striatal circuits drive cocaine seeking and taking. Based on our preliminary data we hypothesize that enkephalin released from D2-MSNs acts on MORs in axon collaterals of D1- and D2-MSNs to suppress intra-striatal GABA and facilitate cocaine reward. We will test this using operant cocaine self-administration procedures in MSN-selective MOR and enkephalin knock- out mice. We will determine how intra-striatal GABA transmission from MSNs contributes to cocaine seeking by selectively inhibiting D1-MSN or D2-MSN striatal axon collaterals using a novel Gi-coupled opsin during a cued cocaine seeking task. Successful completion of the proposed research will elucidate how endogenous opioids regulate intra- striatal GABA transmission, and how cocaine impinges upon this mechanism to affect circuit activity. Moreo- ver, this research will provide novel insights on the mechanisms of cocaine abuse by establishing a link be- tween opioid-mediated regulation of intra-striatal GABA transmission and cocaine seeking and taking. This research will also lay the groundwork for our long-term goal of determining the circuit mechanisms driving synergistic reward when opiates and cocaine are co-abused.