A neural circuit for comorbid depressive symptoms in chronic pain.

Comorbid depressive symptoms (CDS) in chronic pain are a common health problem, but the neural circuit mechanisms underlying these symptoms remain unclear. Here we identify a novel pathway involving 5-hydroxytryptamine (5-HT) projections from the dorsal raphe nucleus (5-HT) to somatostatin (SOM)-expressing and non-SOM interneurons in the central nucleus of the amygdala (CeA). The SOM neurons project directly to the lateral habenula, an area known involved in depression. Inhibition of the 5-HT→SOM pathway produced depression-like behavior in a male mouse model of chronic pain. Activation of this pathway using pharmacological or optogenetic approaches reduced depression-like behavior in these mice. Human functional magnetic resonance imaging data showed that compared to healthy controls, functional connectivity between the CeA-containing centromedial amygdala and the DRN was reduced in patients with CDS but not in patients in chronic pain without depression. These findings indicate that a novel 5-HT→SOM→lateral habenula pathway may mediate at least some aspects of CDS.