PROJECT SUMMARY Post-Traumatic Stress Disorder (PTSD) is a devastating neuropsychiatric disorder that develops after trauma. The expression of debilitating fear toward stimuli previously associated with trauma even after they no longer pose a threat is a core pathology of PTSD. Such maladaptive fear is caused by an inability to learn that the stimuli that had been previously linked to trauma are no longer threatening when presented in safe contexts and with no aversive outcome. These deficits in extinction learning are a highly prevalent symptom of PTSD and significantly hamper quality of life. Efforts to reduce deficits in extinction learning have focused on understanding the contributions of regions like the amygdala, prefrontal cortex, hippocampus and periaqueductal gray to this process. Despite progress made from this focus, sertraline and paroxetine are the only FDA-approved treatments for PTSD. These drugs are serotonin selective reuptake inhibitors and antidepressants. As such they improve mood-related symptoms of PTSD but do not directly address learning- related symptoms like deficits in extinction learning. With 24 million Americans living with PTSD, there is a need for new therapeutic options to treat deficits in extinction learning. Dopamine plays an important role in extinction learning and drugs that increase dopamine levels like methylphenidate and MDMA improve extinction learning. However, these drugs are not specific to the dopaminergic system and could result in substance abuse disorders, in part, via their action on dopaminergic cells in the ventral tegmental area. In this proposal, we propose to study whether dopaminergic cells in a sub-thalamic nucleus called the zona incerta (ZI) can reduce deficits in extinction learning via dopamine-mediated signaling. To test this hypothesis, we will combine auditory fear conditioning in mice with pharmacological, molecular-genetic, viral-mediated circuit tracing, optogenetic and chemogenetic methodology. More specifically, we will trace the connectivity of dopaminergic cells in the ZI, manipulate the activity of these cells and perturb function of specific dopaminergic receptors during extinction training while examining the consequence of these manipulations on extinction learning. Additionally, we will examine how these dopaminergic cells respond to extinction training after exposure to stress ? a factor that impairs extinction learning. Successful outcomes from our work could highlight a novel function for dopaminergic cells in the ZI in modulating fear-related extinction learning. Our results may have translational impact by suggesting that stimulating ZI-located dopaminergic cells and administering dopamine receptor agonists during exposure therapy may improve extinction learning and reduce maladaptive fear that accompanies PTSD.