Deep brain stimulation (DBS) of the ventral capsule/ventral striatum (VC/VS) reduces symptoms of intractable obsessive-compulsive disorder (OCD), but the mechanism of action is unknown. OCD is characterized by avoidance behaviors that fail to extinguish, and DBS could act, in part, by facilitating extinction of fear. We investigated this possibility by using auditory fear conditioning in rats, for which the circuits of fear extinction are well characterized. We found that DBS of the VS (the VC/VS homolog in rats) during extinction training reduced fear expression and strengthened extinction memory. Facilitation of extinction was observed for a specific zone of dorsomedial VS, just above the anterior commissure; stimulation of more ventrolateral sites in VS impaired extinction. DBS effects could not be obtained with pharmacological inactivation of either dorsomedial VS or ventrolateral VS, suggesting an extrastriatal mechanism. Accordingly, DBS of dorsomedial VS (but not ventrolateral VS) increased expression of a plasticity marker in the prelimbic and infralimbic prefrontal cortices, the orbitofrontal cortex, the amygdala central nucleus (lateral division), and intercalated cells, areas known to learn and express extinction. Facilitation of fear extinction suggests that, in accord with clinical observations, DBS could augment the effectiveness of cognitive behavioral therapies for OCD.prefrontal cortex | anxiety disorders | posttraumatic stress disorder | phosphorylated extracellular signal-regulated kinase | accumbens D eep brain stimulation (DBS) is a neurosurgical technique that has become the standard of care for movement disorders (1-3) and is under investigation in major depression (4-6). In DBS, chronic high-frequency stimulation of specific sites reduces symptoms in medically intractable illness. DBS of the ventral capsule and the adjacent ventral striatum (VC/VS) has been used to treat refractory obsessive-compulsive disorder (OCD) in the European Union and the United States (7-10). Little is known about how DBS acts in OCD, emphasizing the need for translational animal studies. A prominent feature observed in most OCD patients is repetitive avoidance behaviors to perceived threats (11,12). The persistent avoidance in OCD suggests a deficit in circuits that regulate fear extinction (11). Given the strong parallels between fear circuits in rodents and humans (13), well-characterized rodent models of fear conditioning could shed light on DBS mechanisms in OCD and related illnesses that feature pathological anxiety.The DBS target for OCD is the VC/VS (14), which contains fiber bundles interconnecting cortical areas implicated in fear extinction, such as the ventromedial prefrontal cortex, the dorsal anterior cingulate cortex, and the orbitofrontal cortex (OFC), with subcortical areas implicated in conditioned fear, such as the amygdala (15). Previous studies in anesthetized rats (16,17) have shown that DBS of the VS modifies the excitability of the OFC as well as prelimbic (PL) and infralimbic (IL) prefron...