Convergence of Pro-Stress and Pro-Inflammatory Signaling in the Central Noradrenergic System: Implications for Mood and Anxiety Disorders

Mood and anxiety disorders are heterogeneous psychiatric diagnoses affecting millions. While the disease etiology is complex, various risk factors have been identified, such as stress. Stress is a neuroendocrine physiologic response to a stressor that promotes organism survival through adaptive processes and behavior. The central stress response, which drives behavioral and physiological change, is primarily mediated by activating the hypothalamic–pituitary–adrenal (HPA) axis. In addition to its effects on the HPA axis, stress activates the locus coeruleus (LC), a bilateral brainstem nucleus that projects broadly throughout the central nervous system and releases the catecholamine transmitter norepinephrine (NE). The combined activities of the LC–NE system and HPA axis work synergistically to produce timely adaptive physiological and behavioral responses to stress. While advantageous in the short term, chronic stress exposure can lead to HPA axis and LC dysregulation, which are thought to contribute to the etiology of several neuropsychiatric disease states. Notably, recent studies have also implicated neuroinflammation mediated by microglia as a risk factor in mood and anxiety disorders. Despite their combined association with mood and anxiety disorders, the potential links between stress and inflammation, and possible interactions between their respective signaling cascades, have not been well-explored. This brief review aims to summarize how LC is uniquely positioned to respond to both pro-stress and pro-inflammatory cues, and how their convergence in this site may contribute to the development of mood and anxiety disorders.