Alteration in serotonin (5-HT) system is involved in the pathophysiology of stress-related anxiety disorder. Medications that modulate serotonergic transmission are widely used to treat anxiety disorder. Recently, evidence from animal studies indicates that endocannabinoid (eCB) system plays an important role in the regulation of stress responses and anxiety related behaviors. Genetic deletion or pharmacological blockade of type 1 cannabinoid receptors (CB1) results in a marked increase in anxiety-like behaviors and stress responses. These behavioral effects are mediated in part, via the modulation of the 5-HT system. However, the cellular and molecular mechanism by which stress modulates eCB signaling in 5-HT neurons has not been characterized. We have conducted preliminary studies to examine the impact of acute stress hormone corticosterone and severe stress on eCB signaling in dorsal raphe (DR) 5-HT neurons. We find that acute corticosterone enhances eCB synthesis/release, which in turn modulates glutamatergic transmission to DR 5-HT neurons. Our results also show that exposure to severe stress induces a down-regulation of eCB signaling in DR 5-HT neurons 24 hours later. This functional adaptation of eCB signaling in DR 5-HT neurons may play a role in stress-induced anxiety disorder. The long-term objective of this application is to delineate the cellular and molecular mechanisms by which stress modulates eCB signaling in DR 5-HT neurons. In this application, we plan to use a combination of electrophysiological, pharmacological and neurochemical approaches to 1) test the hypothesis that glucocorticoids acutely enhance eCB synthesis/release in DR 5-HT neurons, 2) elucidate the signal transduction mechanisms by which glucocorticoids enhance eCB signaling in DR 5-HT neurons, 3) to determine the mechanisms of severe stress-induced down-regulation of eCB signaling in DR 5-HT neurons. Given the role of 5-HT and eCB systems in the regulation of stress related behaviors, the results from the proposed studies will better our understanding of the etiology of anxiety disorder and may contribute to the development of more effective anxiolytics.