Project Summary Exposure to chronic stress has been implicated in numerous psychiatric conditions such as PTSD, depression, addiction and anxiety, and exposure to stressful experiences are one of the most common causes of drug relapse. The Bed Nucleus of the Stria Terminalis (BNST) mediates many anxiety- and stress-responses that drives relapse and the addictive cycle characteristic of substance use disorders. Post-synaptic Gi-coupled GPCR signaling in the BNST (via Gi-DREADDs and α2A adrenoreceptors on non-noradrenergic Adra2a+ neurons) is sufficient to increase neuronal activity and drive drug-dependent behaviors, while pharmacological blockade or avoidance of post-synaptic receptor activation decreased neuron activity and cocaine-dependent behavior. The Gi-coupled metabotropic glutamate (mGlu) receptor subtype 8, implicated in numerous psychiatric conditions such as substance use disorders, is co-expressed on this population of Adra2a+ BNST cells. Efforts targeting adrenergic signaling to prevent relapse have been largely unsuccessful, highlighting the need for innovative therapeutic targets. Interestingly, systemic mGlu8 activation induces cfos expression, a marker of neuron activity, in stress-sensitive brain regions, suggesting that mGlu8 modulation may be a means by which to alter BNST activity to prevent stress-induced relapse. In Specific Aim 1, I will begin to investigate the effects of mGlu8 signaling on measures of BNST activity using convergent anatomical and pharmacological mapping strategies. In Specific Aim 2, I will assess the effect of BNST mGlu8 KO on gross anxiety measures as well as in our cocaine conditioned place preference and reinstatement assays to test the sufficiency of this receptor in driving stress-sensitive behaviors. Furthermore, I will analyze the stress-induced activity of DCPG-sensitive neurons through the use of fiber photometry and in vivo optogenetics. Together, these aims will examine our hypothesis that mGlu8 positively modulates the activity of a pro-reinstatement population of BNST neurons and that mGlu8 signaling is functionally necessary for cocaine reinstatement. In doing so, we hope to identify novel therapeutic targets that will serve to decrease the instance of stress- induced relapse and improve the treatment outcomes of the patients currently struggling with substance use disorders.